IMHO someone did some experimenting on the coin with some type of chemical or acid to see what would happen! I say this because while they treated the entire Reverse with whatever chemical or acid, they did not do the same on the Obverse. They treated everything on the Obverse with the substance including the "P" Mint mark but only treated around the Date, around LIBERTY and by staying a few millimeters within the rim, they only treated the bottom of the letters of "IN GOD WE TRUST" thus the Copper is still showing. If the condition was done by something in the environment, then nothing on the coin would have been spared and everything would have the blackish appearance.