Black stuff coming up from sink in powder room, husband put something down from home depot did not work I put bleach and still comes back not as much but still there

Most likely, you have blocked drain: remove P-Trap, insert small hand snake and clean the pipe as it enters the wall.

Also, the blockage may be in the sink drain itself. Here, remove P-Trap and pop up piece. Look down into the drain. If you cannot see through than it needs to be cleaned, too.

And here is another image how to unclog the sink drain, good luck.
John

That much I know. I can clear it. What I need to know is, "What is it?" Can it be that "black mold" that everyone is afraid of?