The water probably isn't bad it's just dirty. We're getting a lot of posts about black specks lately. Try to isolate it. Is it only the tub valve? and is it both the cold water and the hot. If it's all hot water in the house then I think it's flex connectors at the water heater. If it's just at the tub then I think it's your washer coming apart. But we don't all agree on the source. It just takes a little bit of black to contaminate a lot of water.