The TIE bombers were not the only blue-looking TIEs in ESB. The regular TIE Fighters looked blue in some parts too. I do not think it was intentional, and I think that the original intention was that they were going to be gray. However, due to some errors in lighting the blue screen, it's possible that the blue ended up reflecting off of the gray TIE models and they ended up looking blue, and since color correction was limited back in 1980 they left them that way.

Whatever the cause, for consistency's sake the TIE Fighters should not be blue, and if you're going to make all of the TIE Fighters gray then you might as well assume the TIE Bombers should be gray, too. Blue is out of place for the otherwise colorless imperials, anyway.