In my opinion, if you want a black bumpers then you have to make it part of the colour scheme for the whole car. Like what the bloke has done with this Z (and many others) where there's the black rear spoiler, mirrors and flares. I'm not sure if I'm painting my front spoiler yet, I'll have a black rear spoiler, black mirrors and a couple of other black items.
If you just had a black front spoiler, I dunno, depends on the car. Doesn't Bruce have a black front bumper on his? It looks pretty good.