Some might describe me as a plating expert. Those dark patches on either side of the snap dome looked like they lined up with the two contacts on either side of the post in the center. I believe those contacts and their respective legs leading out of the body are a single piece and, speculating further, I believe those legs are tin-plated which would mean the two contacts on the inside are also. The black stuff is probably tin oxide as a result of fretting corrosion (its the result of repeated, low amplitude motion between the snap dome and the contacts). I don't think I would say it's bad plating, its just a very cheap way of doing it, both because tin is relatively cheap and because it enables the leg/contact piece to be stamped and plated as a single piece.
Of course I'd have to throw it under the XRF to make sure but the shipping costs....