Slimey black is usually iron and/or manganese reducing bacteria. It produces a ‘sulfur’ odor. The bacteria live in the earth and come in with the water from the well. You may find the same slime in your water heater if you drain it. You may also develope a hot water only odor if your temp is below 140f.
Contact water treament dealers in yoru area for identification and cures.
Quality Water Associates