Interesting as we're right in the middle of deciding blue or black and have the two-sided backing tacked to the tank with blue showing right now. Right now, the Mrs is leaning toward the blue, me toward the black (Mainly because it makes the overflow, PHs, etc blend in, the blue strikes me as too bright).
Actually, I think a very dark blue would be best, thinking about painting the blue side of the backing with a darker blue.