I thought it would look cool, but after a trip to our favorite LFS, we quickly decided that I wasn't planning to vacuum it every day and if I didn't, it would look bad most of the time. So the black is out! =)
Black sand = scratches on glass. That is the #1 reason it isn't used nearly as much as white. There is a newer type called Black Tahitian Moon sand that is supposed to not have the rough edges and be as safe as white aragonite sand.
There are some sand dwelling corals that look great sitting on black sand (mostly various brain corals).
I've never put black sand in a sw tank myself, but one of the LFSs here has a coral holding tank with black sand in it. The tank itself definitely appears more dim (visually), but many of the corals look great on the black sand.