Black, foul smelling greasy oily residue is the result of bacterial decay, waste digestion, and stagnation normally associated with septic tank, or broken sewer line stagnation, and results from a cross connection somewhere nearby. Black very fine sandy gritty (but not mud) residue without a foul odor and greasy oily consistency could be iron pyrite, iron oxide, black sand, caused by heavy deposits laying in the City water supply line close to the end of a 12 inch service where a much smaller return line or fire hydrant (which is not regularly flushed and maintained) is placed, and is close to where your house supply is connected. Take a clean sample of it to your City Water purveyor or Health Department and ask for written Laboratory Analysis to ensure the water is not unsanitary or unhealthful.
: I have copper lines connected to city water. I am getting a lot of black residue in aerators and toilet bowls. Redisue is not rust, it is almost like mud. What could this be?