How to get rid of black stuff in bathtub drain?

My inlaws never use their bathtub (only shower) and now were visiting and our son is using the tub but every time we stop the drain this black stuff is coming up. They never really bother cleaning down in the drain because it's never been a problem but we can't seem to be able to get rid of whatever it is - probably some build up of grime, possibly mold, lime, who knows!! Any idea on what we can do? We tried using pipe cleanings but we can't get it out. I thought maybe there's a type of drain cleaner that we can pour down there and it will do the work for us? Something safe please! Thank you!