Black film from furnace/AC

I would be concerned that exhaust is not venting properly and the soot is getting into the fresh air side of heater. Have unit checked for carbon monoxide presence in fresh air ducting. Heaters use a heat exchanger the is heated by the furnace and fresh air is blown through yje exchanger to be warmed before it gets out the vents. If the exchanger is damage or rusted through the good and bad ai can exchange creating a serious/ life threatening condition. I recommend professional inspect and repair and have a carbon monoxide detector in place and functioning. Be safe and good luck