Black stuff on tooth?

I was checking out my teeth and there was some black stuff at the area of the tooth where two teeth meet. 

I panicked and thought it was a cavity

I brushed the black stuff away fairly easy with just a toothbrush

This is what’s left of it. It’s the dark dot on the left of the pic. 

Is it a cavity, something stuck in my tooth or something else?

I know you aren’t dentists but I’m asking for people who have had similar things/cavities