Gunmetal...You know you want to...hehe :naughty:
Actually, on a black car, a gold/bronze color may set it off quite right, maybe something like this?
If you decide not to have them powdercoated, Dylan (cwapface) has a rockin'
paint your wheels how-to in the Reference section, which may save you some money--however, if I had it to do over again, I'd have them powdercoated, as long as they could do the color I wanted...
Personally I'd recommend against black, because the black of the rim will never quite match the black of the rubber unless you put tire shine on every 3rd day. Unless you went with a flat black--that would be sweet, but most likely a pain to keep clean...
EDIT: Come to think of it, I'm an asshole. Black rims would probably look pretty sweet if you had a polished lip (or even just a machined uncoated lip) to break up the black of the rim and the black of the tire. Just a thought...