I understand why people like the natural look of the blue background, but I prefer black as I find it really turns the tank into a centre piece and makes the colors pop. I considered the same thing with my tank, but my fw tank has a black background, and the overflow in my tank is also black (corner though), so I figured oh what the h, just paint it black and be done with it. So, the answer is simple...keep the overflow, do a Herbie, and paint the back black! Just like me!!!
Although I don't think it would look bad if you kept the black overflow and painted the back of the tank blue. I think it would look just fine.