I am using black construction paper on the back. I figured if I didn't like it I can change the paper. I like the black and since the coraline is finally growing I probably wont see the black anyway.
_________________ Honda XR650L
On my current tank I have black. I think the black makes the corals/fish stand out a bit more when you look at the tank, especially at a distance. The black back seems to make you ignore the back all together. On my old tank I had a blue background. It actually was very dark blue at the bottom and gradually faded to a lighter blue at the top. It was kind of cool. I think the blue gives a little bit of an element of depth to the look. Personally, I liked the depth of the blue a little bit better than the black. But when it's all said and done I think it's just personal preference. I will agree with Karl though, blue doesn't look very good with corraline or other spots on it, while black is okay.