I am thinking it was ran through a washing machine, stayed in the pocket of the clothes and the clothes were air dried ? But I think the coloration is caused by water. But at any rate, I believe it is environmental damage.
I hooked a pair of jeans while fishing a few years ago. There were some silver coins (quarter, 2 dimes) a nickle and a few cents in one of the pockets. The silver was so black, I could not even read the dates ! Amazing what exposure to water, dirt, soap, etc will do to a coin.