Seriously??? Ok its environmental damage. Been subject to either dark, damp or humid condition in cardboard or thick paper wrapper. Ive got some steel cents its happened to also. Got them out of a roll like that. The rainbow is tuti fruity.
Yep, I've found red, pink, green and blue, so why not black. But I found more black, so I guessed it could be naturally occurring, and thanks to everyone's replies now I'm confident. Thanks.