Looking at it from a punter's point of view, I appreciate it when the performer(s) look as if they've made a bit of an effort and haven't just thrown on the first thing that came to hand. This doesn't mean it has to be formal, but it should look deliberate. So when there's more than one performer, it looks good if there's some evidence of collusion, so that you get the impression they did at least discuss what they'd wear (and even better if they end up with a pleasing combination of colours and styles, not just 7 shades of black).

The appearance of the act/band is part of the enjoyment of a live gig, and it seeom odd to me when a band take endless trouble over sound-checks etc but look as if they couldn't care less about the visual aspects of the performance.

I'll endorse what Anahata says about black - quite often the background is black, and all you can see of the black-clad performer is the hands and face. Anyway, black seems to say, "I'm not really here, I'm anonymous, don't look at me" (one reason teenagers like it so much). Black is for the sound-crew and the stage-hands. To me, performers come across better with a bit of colour and brightness and individuality.