I'm not a big fan of blue myself, I think it will look weird with the different colors on the sides. I did see a really cool one on RC, that started a lighter blue on top, and when down to darker and finally black. Looked really sharp.
That said, if your not 100% sure pick up some vinyl at a local sign/tint and use that. Much easier to remove.
I painted the back of my 75 black. Make sure you clean the tank with a non-oil based cleaner. Make sure there is nothing on the back of the tank. When I did mine, I had just got it back from the glass shop which had some kind of oil based cleaner on. I also painted over a little speck of something I didn't see. First time I used the mag float it ripped off leaving a clear mark in the back
