I agree with the above, black is great. It shows off the colours of the fish.. and makes everything else like plants stand out too. All my tanks have matt black backgrounds. I specify matt-black because there's nothing worse that light glare off a shiny black background.
That being said, I've seen some awesome setups with backgrounds other than black. Very dark brown goes well with black water setups and very light grey with white water rapid setups.
Not a fan of blue though for any freshwater tanks, personally..