Old pipes can begin to corrode after many years of use. When they do, small, irregular pieces of pipe can break off and wind up in your water.
If you see black specks when you turn on your hot water in the tub, shower, or sink, your water heater is likely corroding.
Are the black specks in your water small and rubbery? These could be disintegrated pieces of a rubber gasket, washer, or flexible supply hose.