I've done blue, gray and black backgrounds and black is the only one that I have ever liked. The black just seems to make the plants really stand out. I can't explain it, but the blue and gray just looked awkward to me. Once the tank is planted and established, you really won't see any contrast between the soil and the background.
I'm trying to find a pic, but I saw a tank that had the back painted in a fade from navy at the bottom to almost white at the top, sort of similar to a sunset without the reds. It was pretty nice looking. If I can find a pic I'll add it here.