Looks like you’ve worn the plating off, you can probably remove the black smudge, but nothing is bringing the color and finish back.

Dremel, polishing paste fine and very fine, polishing wheel and wobbles, and time. Test it on a small part, maybe under the ear piece, if you can pull it. Works with a bit of heat, mostly. Not too much heat, though. More hair dryer than a heat gun. If the discoloration is under the plating it’ll be silver in the best case, if it’s above it should work.