Depends on your definition of black. Sounds to me like the motor has got very dirty, that this might well have caused rubbish to get on the end of the rotor and the sensor plate, and eventually something has stopped in the sensor-plate department.
Despite the claims to the contrary, BL motors must be cleaned regularly like any other motor, as they get dirt in them and damage can be caused. Hope you find that a damn good clean out and some TLC gets it back going again! HTH
