Not sure. I wouldn't use them. The link below is to a discussion of this.

FataMorgana

Here is a link that might be useful: Powdery Mildew

Do they smell 'off' and mildewy? I've not yet met a mildew or mold that didn't give off some sort of scent.

As I do not know where you are located you could be in the western states, in which case what you've encountered may be a normal trait of the Blue Elderberry fruit. I don't know if its possible to have multiple links posted at the bottom, so pardon the ugly url list. Here are some photographs of what the berries can look like. I should have thought to take photos of my own when processing.

http://oaklandlocal.com/sites/default/files/i/elderberries%201.jpg

http://stitchandboots.files.wordpress.com/2009/10/elderberries-macro-2.jpg?w=450&h=390

Note how in the first image, not all of them appear with the blue or whitish powder on them. They can all have it, or some clear black ones can be present - even entire separate clusters of them on the same plant. It does not rub off easily, and you'll likely just tear the skins off if you try. Something I found on accident that will make it disappear is sun exposure, once picked from the stems (don't know if it works when the berries are left on). If you're unsure if its mildew still, try leaving them in the sun for half an hour or so on a reasonably warm day - if they turn shiny and clear black, it wasn't mildew.

I've also included a link below to some information about the specific species I'm talking about, Sambucus cerulea.

Here is a link that might be useful: Sambucus cerulea - blue Elder