I just feel like having all the plumbing and wiring visible can ruin the aesthetic of an otherwise beautiful tank. I painted the back of my 180 before I set it up and it was really easy. Now, since I am not particularly coordinated, I taped a black background to my 90 gallon to avoid a mess. Fortunately, my 55 gallon acrylic tank has the black background already built in.
Bottom line, whether its black, blue, or a picture of a bunch of tree roots, I really think it makes the tank overall look cleaner.