I recently opened some 3/4" copper tubing before my water softener to add in a sediment filter, and was surprised to see a black greasy substance inside all of the tubing, I notice the toilet bowls get a black film inside them within a day or two after cleaning them, We sometimes have a sulpher odor coming from the hot water, my question is is it safe to use it for all but drinking water? We do have an RO system for drinking water.
Sounds like you have black sulfur. Unpleasant, but not especially unsafe. You could get a water test and know in more detail what you have in your water.
Anecdotally, I lived in a house that had a very bad case of that for a year or two, carrying in drinking/cooking water, and it was never an issue, though if I was away for a week or two the first flow of water from the pipes was solid black until it flushed itself out. I don't miss living in that house.