It is a result of oxidation, almost all silver jewelry would experience the progress for long wearing.
In fact, silver should not be worn with gold jewelry mixed, for they are of different character, just like the dress suit is not suitable to be worn with jeans.
Some style of silver would become black, and it is not necessary that it can be worn only after it is cleaned clearly in that the nostalgic taste is great as well, just like the patches on the jeans.
When the color of silver changing into old black, it is very nice to wear retro style items, cleaning it is not that necessary.
Well, hope these tips would help!