Black aluminum beams are annodized or pasivated that color. This is a dipping process that protects the aluminum from becomming aluminum oxide on your hands and clothes. Natural beams are likely treated with a clear treatment. Painting is not likely to hold up well to chipping and will look bad very quickly as the silver starts to show through. You see the same problem when people make white boats black and other such major color changes. My advice, simplify your life and just clean them well.