Just rub it with some 600 paper and give it a whiz with gloss or semigloss black.
Or replace them.
My car came with only 1, so I made a pair from a flat piece of cf. They lie more flush than the OE's. Used 3M plastic trim and emblem adhesive. Mine was flapping in the breeze, lucky I checked the mirror before it let go. Not that I need it anyway.
I don't think GM designed that ordinary trim piece in a wind tunnel.