I have a low growing coreopsis that stays very neat. It seems to barely bloom, though mine isn't in as much sun as it probably wants.

Actually, I think the Royal Candles Veronica would be awesome there. It's a bluish purple. There are several threads extoling it's virtues right now. Really can't go wrong with them, they have a super long bloom time and always stay nice and neat.

Have found the dwarf, seed grown Kobold Platycodon or Balloon Flowers dependable in well drained soil They come doubled, in blue, pink and white and, while not all Summer blooming, they seem long lived. And what about repeat bloom, dwarf Daylily?

one of the smaller eryngiums? Pinks?

Stokesia laevis/Stoke's aster produces mid-to-late season ruffled blue flowers, has elegant foliage and meets your height requirements. You might also check out some perennial geraniums--there are several available that form low-maintenance mounds that are covered in delicate blooms in pink, blue, lavender & white. Polemonium/Jacob's Ladder produces blue flowers early in the season and is another carefree perennial but it grows a bit taller at 18-20".

Thanks for the suggestions!

Aster Blue Autumn, in the three years I've had it has stayed very short and heathy with pretty flowers. All thou I don't know what experiences others have had with it. So far Royal Candles has been taller.

Yellows? Someone say yellows? Check out Heliopsis 'Tuscan Sun' Should comply with your wish in the sunny spot.