If your tank is planted & the fish colourful it can look stunning.
I started with black gravel & black background, then I changed some of my tanks to playsand, but I'm now thinking of going to black sand
I've got a black background and sand, check out the link in my sig for pictures (you'll want to look later on in the topic for when the black sand was added)
I don't have sand. Instead I have a small 2-3mm unipac black substrate that after a year has kept its color so far. To me, like @FishFanatic04 already said