the black dust is coming from excessive wear on the belt. It may be someone has reduced the diameter of the alternator pulley. to increase it's output at low reves. Check your belt size. Both length and width, these dimentions are stated on the outter surface of the belt. Does the belt slip when running?(makes a squeeling noise until the batteries have increased their charge levels) ceck alignment of the pullies. this should get rid of your dust.