I have noticed that some merchants charge more for a chrome lens than for a black, and to others it's the opposite. I've learned (from experience here) that chrome lenses are usually heavier, as they are made of brass and not aluminum (or something along those lines). Now, the black paint strikes me as a mere fad, same as the black paint MP body, which is designed to fade or deteriorate in the corners for a more worn-out (therefore professional) look.
Can this be the reason? Let's hope somebody here knows about it...