These are all growing & thriving on neglect in my garden:
Platycodon grandiflora/balloon flower
Stokesia laevis/Stoke's aster
Lobelia siphilitica/great blue lobelia
Phlox paniculata/tall garden phlox 'Blue Paradise'
Nepeta faassinii/catmint 'Walkers Low'
Caryopteris/blue mist shrub
Adenophora pereskiifolia/ladybells
Baptisia australis/false indigo
Catananche caerulea/Cupid's dart
Centaurea montana/perennial bachelor button
Campanula persicifolia/peach-leafed bellflower
Aquilegia vulgaris/Columbine 'Winky Blue,' 'Songbird Blue'
Perovskia atriplicifolia/Russian sage

I love the blue of my "Monkshood" showing itself late in the season (October for me).

I really like flowers in the blue to purple range, so I have a bunch of them
I have some of the same blue as Gardenweed.
Platycodon grandiflora (balloon flower)
Nepeta (catmint) 'Walker's Low'
Baptisia australis (false indigo)
Centaurea montana (perennial bachelor button)

And a few others
Baptisia 'Purple Smoke'
many blue and purple clematis, both ones that climb and ones that are medium to tall perennials that don't cling, but may need a cage for support or can sprawl. (If you want specifics, I'll look up names.) They will need mulch and watering if there is a long dry spell, but they are worth it to my mind.
Gentiana septemfida (crested gentian)
Veronica 'Georgia Blue' and 'Waterperry blue'
Siberian Iris, both the regular type and several named varieties in blue and white
Campanula - several types: 'Blue Clips', 'White Clips', portenschlagiana, portenschlagiana 'Resholt Variety', poscharskyana. All of these are under a foot in height
Bearded Iris
Japanese Iris (though they like some moisture)
Peonies come in white, but not purple

I have grown Lobelia siphilitica (blue lobelia) and am still pulling seedlings out of the lawn, from under shrubs, in the veggie garden, etc, etc almost 10 years later. I wouldn't recommend this plant unless you are really, really good about deadheading or you know that it doesn't self-seed as vigorously around you as it does for me.

Also, don't forget the various perennial bulbs, many of which will extend your season of bloom. Bulbs in the white and/or blue/purple range include Scilla, Chionodoxa, grape hyacinth, tall alliums, crocuses (both spring and fall blooming types), hyacinths, reticulated irises, some white daffodils like 'Misty Glen', Galanthus (snowdrops), white lilies, Leucojum, and white Darwin hybrid tulips.

Campanula trachelium Bernice
Salvia Caradonna,
Delphinium elatum,
Gentiana asclepiadea,
Scutelaria incana,
Aster amellus Sonora,
Aconium carmichaelli, Aconitum Bicolor,
Tall Bearded Iris Sapphire Hills, Sultry Mood, Best Bet
Iris sibirica Blaumacher, Prussian Blue, Big Blue, Berlin Ruffles
Echinops ruthenicus

OP is in Wisconsin so I listed only those blue flowering perennials I've been happy growing. There are lots that fit the color but have drawbacks which is why I didn't include them. Delphiniums are primadonnas where I am and so far they've struggled to survive, let alone bloom. Eryngium/sea holly did okay and I love the flowers but the foliage looks ugly soon after they bloom, too early in the season, for the rest of the season. They got shovel-pruned. Gentian grew so slowly I figured I'd be pushing up daisies long before they bloomed. I'm giving aster a try this year for the first time. Siberian iris quickly forms a concrete root mass that can't be dug out with anything other than a backhoe...or dynamite. Love the flowers but they don't last and I decided not to list them based on what I consider their bad behavior. I love the look of aconitum but not its toxicity. I should have included Japanese iris because they are gorgeous and from my experience thus far, not apt to become a nuisance.

nhbabs - thanks for the heads-up on lobelia siphilitica. It's a new-to-me perennial which I hope doesn't become a nuisance. I keep plenty of vinegar on hand but it's always wise to check before planting. It's growing where it doesn't have its "ideal" conditions which I hope keeps it in check.

Haven't grown it but have heard that Echinops/globe thistle is a thug on this side of the pond.

Echinops stays put and the only thing you have to do is to cut the spent flowers, which is easy. It does not like to have 'wet feet' in winter(sure death if flooded for some days).
I would not recommend phlox Blue Paradise, unless you really love to stake. The worst flopper around among the tall phloxes.

Salvia azurea and salvia black and blue are both perennial for me in 6b and provide great true blues with long bloom times. Monkshood will give great blue/purple for the late summer and fall.

KimKa

-Eileen, Echinops bannaticus 'Blue Glow' has been quite a pest for me. Really annoying bugger. The seed heads shatter very easily so you have to be vigilant about cutting them off right after bloom. Any seedlings to do appear are a nightmare to dig up, what with the long tap root. I decided to get rid of my big plant this spring and dug it out. Guess what I found come fall? Yep! The thing was growing again. I suppose this is the type of plant that will re-sprout from the roots if you miss any...

A favorite of mine is Hyssopus officinalis 'Dwarf Blue'. Super drought tolerant and not fussy in the least. True blue flowers. Nepeta subsessilis has nice flowers, although I don't find the species all that vigorous...

Don't forget blue foliage! Blue Fescue and Blue Oat Grass (Helictotrichon) are wonderful. There are also various sedums with blue-ish folaige.
CMK

Lots of great suggestions here. I think I have to second the warning about the lobelia syphilitica. I planted some a few years ago and it does seed fairly easily, especially in the damper areas. I have started to pull it and plan to take it out completely. Too bad, as it's a lovely plant. I will add though that the plants that are in drier areas don't spread as agressively.

Love monkshood - beautiful late season color. Also love blue platycodon.

Not a perennial, but one of my all-time favorite blues/purples is salvia farinacea Victoria Blue. Long-lasting, beautiful color, will reseed but not aggressively, and I've even had plants overwinter for me. Great plant!

Dee

I haven't had difficulties dividing or moving Siberian iris, so it must be a soil difference. My Gentiana septemfida are 3 seasons old and in a bit more shade than they might like; they are still small but bloom well in September. I also forgot Gentian 'True Blue' which is tidy and blooms prolifically, but earlier in the season than G septemfida. I've had it in my garden for two years. It's a bit over a foot tall, while G. septemfida is only a few inches tall. Salvia 'Black and Blue' isn't hardy here, so I wouldn't expect it to be in WI, but I buy a few each year when I haven't been able to winter it over in a cool area in a pot. Stunning flowers.

Christin - thanks for the info re Echinops since it bears witness to what I've read/heard from others. I'll pass on it for the very reason you state.

Dee - my blue lobelia is growing where it gets no supplemental moisture, nothing but what Mother Nature doles out, however I'll keep an eye on it for volunteers. I'm also impressed with salvia farinacea, even though mine hasn't bloomed yet. It appears to be one tough customer!

I may ask the migrant workers to dig out the Siberian iris I unfortunately planted near the foundation on the southeast corner of the house. A neighbor gave them to me and when I planted them, I didn't know they spread so fast. The clump doubles or triples in size every year so it's a little unnerving considering it doesn't have much more room to spread.

Would love to plant gentian or monkshood but haven't found the opportunity up to this point. My focus has been on adding plants for the bees, butterflies and hummingbirds. Might be fun to plant something just because I like it for a change.

All, appreciate the great suggestions! And its great see a little conversation about performance.

I'll be sure to share a few pics in spring when I get a few of them planted. There are a few listed here that I totally forgot about so again, appreciate it!

In addition to many of the blues and purple I share with other posters above,
I am particularly fond of Aster oblongifolius "Raydon's Favorite' ( a taller,
lavender/blue) and it's cousin, Aster ob. 'October Skies' (only 18" and more
distinctly blue). . .both are some of the latest blues you'll find for a northern
garden.

An early season favorite is Geranium 'Johnson's Blue', which is very much more blue than the G. 'Rozanne' you already have - I happen to love it's sprawling habit, weaving amongst other plants and clambering up anything strong enough to support it.

Although it's a tender perennial in our zones (so you DO have to replace it
each season), there is just nothing to compare to Salvia leucantha (Mexican
bush sage) - the flowers are long inflorescences of the richest purple, just
begging to be stroked because they feel just like velvet! Adults are charmed,
kids are fascinated. . .and it's a late season bloomer. Rarely found at
retail nurseries, but there are several sources on-line. . .

Given excellent winter drainage and a little cover, there are also a few
selections of lavender that you could grow there in Zone 5 -.can't imagine
a blue/purple garden without it - besides, the silver fliage would look smashing with your conifers!

Carl

There are a number of Caryopteris rated hardy to zone 5, including this one. Well-drained soil and winter mulching are advisable in colder zones.

I also second the idea of growing temperennial Salvias. S. guaranitica sometimes overwinters here and in addition to having terrific blue flowers also attracts hummingbirds.

Rozanne Geranium and Baptisia 'Purple Smoke' do well for me and although its a very dark pink, its almost purple, I'd suggest Echinacea 'Fatal Attraction'. The Iris pallida with white variagation is another and it goes well with conifers even after the blooms are gone.

tj

I hesitate to suggest these because everyone say they take over but I have not had a problem moving them or removing the ones I do not want, Bluebells.

I really love Agastache 'Blue Fortune'.
They require good drainage and winter protection here.
Another favorite of mine is Scabiosa Caucasia'Butterfly Blue'.

I love Caryopteris for their nice late season blue color that lasts and lasts and there are some nice foliage colors now

Hemerocallis 'Indian Giver' is a true purple that reblooms for me.

I have a veronica Blue Charm which I bought from High Country Gardens probably 10 years ago which has remained vigorous and blooms most of the summer. It is more tolerant of moisture and rich soil than other veronicas, leans more to purple than blue and doesn't flop. Another favorite is stokesia Omega Skyrocket, the flower stems are taller, 3-4 feet and it is such a beautiful color, though it has lost some vigor over the past few years. Of the salvias, I like Caradonna the best, not just purple flower spikes, but the stems are purple also. May Night does well here, very early blooming. Also salvia farinacea Victoria Blue, is hardy here and seeds about. Two of my favorite blue/purple are larkspur and nigella. Biennials, but seed about easily so seem like perennials.

For me the best true blue is ---Salvia "Blue Angel". It is a perennial for me here in Oregon.
{{gwi:285271}}

Two more outstanding purple/blues that haven't been mentioned are tradescantia 'Sweet Kate'. Makes a nice edger, has contrasting gold foliage, long blooming dark purple and amsonia 'Blue Ice'. Shorter, bluer, with thicker foliage than hubrichtii, which is also nice if you prefer subtlety.

omphalodes 'Cherry Ingram'
Lithospermum 'Grace Ward'
Linum narbonense and L.perenne Saphyr
Anchusa Loddon Royalist
Delphinium belladonna Cliveden Beauty
Campanula Telham Beauty, Kent Belle
Aster Frikartii 'Monch'
Elymus magellanicus
pulsatilla halleri
iochroma
polemonium Lambrook Mauve
dierama blackbird
meconopsis - many, especially Lingholm

Hi Whaas, I've looked up some native plants that I grow, or hybrids of natives, and these are listed for z5 or WI. Sorry, I don't know about soil.

Phlox divaricata Louisiana blue (purplish blue) blooms first in the spring and then phlox pilosa (pink & purple varieties). Princess Diana clematis (hot magenta, large flowering hybrid from crossing with native texensis), Monarda didyma (our native one is a dark mauve purple & listed for z5).

I've read about native Viola pedata bird's foot violets and lots of clematis that are purple shades. There was a famous Polish breeder of clematis, Brother Stefan Franczak, so his hybrids would be cold hardy for you. Clematis on the Web lists his large growing "Polish Spirit" as deep purple blue, "General Sikorski" as mauve blue, "Emilia Plater" as blue violet and his pale "Blue Angel."

Here is a link that might be useful: Article on Brother Stefan Franczak

All, thanks for the great suggestions!

The beauty is that I can repeat the color scheme but have alot of choices to work with.

I haven't heard of half these plants so I have my work cut out for me.

Not sure why I mentioned May Night Salvia in my original post. I meant Blue Hill Salvia...much, much nicer color.