You might try:
Is the black stuff slime or sandy grit? If slime:
- Drain your water and heater tanks completely to the bilge
- Partially refill and drain again about 3 times
- Fill and let sit for an hour or two with about 1 cup of bleach for each 100 gallons while flushing through the system.
- Drain and repeat #3.
- Add a "purifier" and follow system odor - assess need to retreat in a month or two.
If grit, flush tanks several times until grit gone. You may want to buy a hose end filter prior to refilling as problem may be source of water