I like the idea that the tank looks as natural as possible, and I've heard or read horror stories of black gravel's fading to prison grey over time. The black is achieved with some sort of dye, as well.
Still, I can't deny that when I got a few black gravel stones hitchhiking home with a plant, I liked the look of them around the base of it so much that I asked for some more when I bought more plants from that LFS later.