Hayslope- good point about oxidation. I know little about pewters reaction, and I never liked chemistry, but I have tried to find some information on the net.
The oxidation is a result of the lead in pewter. This causes a darkening. This is a typical pewter finish. Oxidants can be chemicals or simply oxygen. Can pewter oxidize beneath finish, cut off from air? I don't know, but I am thinking not.
Perhaps the finish was supposed to look like oxidized pewter?
I believe this picture is proof that the black is not from oxidation. It is a 61 Grizzly. NO finish over coin. Look at the rim of the coin? Is it black? NO. Is there black on the coin? YES. Is it from Oxidation? NO.
My guess is that they painted the coin black before installation and then sanded/polished/scraped the coin to leave some of it in relief. Another, TradGanger proposed something similar above.