iT IS nice for the fish to have a place behind the rock where they can go hide. any fish store has backgrounds they sell. Black makes the coral stand out more.
my back piece of acrylic is black on my tank. The tank are much more comfortable in the back of the tank, than they were in my last tank which did not have this.
I picked up a large piece of foam core at Office Depot and spray painted it. I like the foam core because it is rigid and does not show ripples. Also my tank is 30" high and I could not find an background that big.