Black tooth?

I had a very wobbly tooth that fell out. The tooth next to it is now extremely wobbly and is hanging by one root in my mouth, and won't fall out. It doesn't hurt except when i pull at it obviously. I can twist it round that's how loose it is. As I twisted it onto the backside I noticed I have a black part on the backside of one of my teeth. What is it? Should I be worried about it? Please reply I'm getting worried and starting to panic. I'm 11 and a girl. If that helps at all.