I've seen a widebodied S4 in black and it looked fantatsic. He had gone the whole hog with a lot of RS4 engine parts, RS4 dials, steering wheel, gear knob, etc. He'd also added some carbon trim on the car, which did look good.
I agree with Siena though, it was just too subtle in black.
I love the fact my S4 is stealthy, but if I was going to fork out that much money on what is effectively a cosmetic change, i'd want people to notice!