IF you painted the panels, back to black will only make it so you can't fix the paint. I'd take the panels off the car, sand them down till they look frosty and ugly, paint them with several coats of the color you were looking for, which would probably be flat black, but before that, make sure you lay down a flexible primer so that when the panels flex, the paint will still stick, then lay down several coats of your color, and then add a nice satin or crystal clear, clear coat, and add two or three more, wet sanding inbetween coats, and then when you put the last coat of clear on finish it off with a piece of #000 steel wool, and that will clean off any bumps, and you should have some beautiful flat black bumpers and it will look just fine. don't try to "fix" it. Re-do it.