I say paint it!! I think all tanks should have the back wall painted. i like the shade of blue you have picked out. I'm not a fan of the lighter blue that a lot of people use, especially fish stores. Personally, I prefer a black background. As much as I understand the idea behind the blue background looking more natural I don't like how the blue takes away from the purple coralline and the blue fish. So I opt for black which I think makes all the colors jump right out, and it hides overflow boxes really well.
