Blue and white twitch jigs worked really well for a buddy last week, even after first light he still managed to get some coho to take so I don't see why a blue fly wouldn't work well.
Black is just something I've used years ago especially on the Cap, works really well in certain pools and I just feel that with brighter conditions the color doesn't stand out to much sort of like a bright light bulb when compared to colors like hot pink or chartreuse.
Hook: L11S-3H size 4-6
Head: Medium to large bead head in either copper or black
tail: Black crystal flash about a little longer than shank length
body: Black palmer chenille medium
collar: Black Shlappen trying to keep most of the feathers about half the length of the hook shank with 2 wraps
I don't fly fish the Vedder much either but once and a while when you can find a nice run without to many people it can be fun and relaxing, a little extra bonus for being able to hook a coho or a chum on a fly that is no bigger than the first digit of my thumb.

I would have thought that most of the time the reaction that coho give for striking small flies or even large flies is more of a protective/ aggression towards predators or their eggs like small fish, leeches and other such things that lurk below.
It's possible it's a feeding reaction to seeing small fish like sticklebacks.