I'm so confused- what "micro-organisms" are you talking about? And you bought a Gucci dogtag necklace just for the chain? You know you could have bought a silver chain by itself from a jeweler and saved quite a bit of money. Just my two cents.
Oh wow....that is a crazy story. But sweetie...if you are worried about Germs...the sink is the worst place to be!!!! Try some rubbing alcohol...or some sanitizing lotion or something.
As for black on the silver cloth. YES...the black is dirt/tarnish coming off!