We're having this conversation in my band right now as our photo shoot is coming up next Sunday. We're a bit resistant to the idea of a uniform as such, whats important to us is that we're distinctive and memorable - our individual personalities will count most for that. But the girls all have such divergent dress senses that its hard not to look a little chaotic.
For example, one of our singers likes her bangles and jangles, another is more urban chic and a third tends towards gamourous. Its hard to reconcile those tastes into a look as such. However, the Spice Girls did a pretty good job of it and I'm still trying to crack their formula. I think the lack of accessories helped a lot, they didn't generally wear patterns onstage and each of them chose a look that didn't overlap with any of the others.
Of course, in some ways the singers are the easy bit. The musicians are a little harder because we are in more of a supportive role and so our personalities aren't so crucial. I completely agree about black and the backdrop - we've had this experience ourselves mainly because the lighting was rubbish at most of the venues. If you look at some of the Jools Holland HipHop/RnB DVD acts most of them are looking good wearing black. I suspect this is because the lighting in the studio was a lot better than we've had on stage.
Wearing something lighter in tone definitely helps lift the band out of the background a little.