Maintain an Air Conditioner: Why does my window air conditioner blow out black debri? It is dust and dirt from operation. You need to pull the inside frame and works out of the cabinet and inspect the inside. Wash and vacuum the indoor blower section, fan wheel and the outside section coil. then dry or let dry ,reassemble and re install. Be careful not to wet the fan motor and bearings wrap in plastic while you wash the other. More at howstuffworks
Kleen-Screen™ Air Conditioner Coil Filter -- improve energy efficiency; protect A/C unit from 95% of most debris that can clog your air conditioner condensor. Easy one-person installation. Dust to clean. About $30.00.