Can hard water stains be black?

My husband and I just moved to a house that the former owners left their washer and dryer in. The washer had a brownish stain from almost the bottom of the drum up that got increasingly darker until it was almost black. I don't know how to know if the stain was from mold or a hard water stain. It wasn't spotty, just a solid ring around the tub. I don't know if running bleach in the washing machine is enough to get rid of the any mold in it, if that is the problem.