The black under the paint is correct, most factory pieces from most manufactuers use black gelcoat plastic these days, My mother in laws Malibu has a scratch bumper and underneath is also black.

 

Our 1999 ML320 chips and peels easily. I believe its from the federally mandated water based paint that came around in the 90's. Its easy to touch up using proper technique, but the bumper is not. Same run around from the dealer (I don't go to that one any more). One wheel peeled early and got Mercedes to replace it.

I now use a bra for my black 2005 E320 CDI and it looks good. Since your ML is black, I would recommend a black bra. Colgan makes a good one.

Cheers