I don't think it is black mold. You would have mold every where else in your housebefore you would have it near the air ducts. Black mold is a type of mold. It refers to the color of the mold. There are tons of types of black mold and very few are actually dangerous. Staccybotrous (sp?) is the bad one that can affect children and elderly. In order for mold to grow you must have a few variables. Mold will not grow if one variable is missing. Mold grows in warm, dark, damp places. It must have a food source. Bacteria on anything can serve as the food source and there must be water or moisture of some kind. Near an airduct the air would be very dry unless you live in a very humid area. MOld begins to grow within 24 to 48 hours of water being introduced. Mold will also 'bite' into the surface of whatever it is growing on. Have you ever seen a bath tub with a window in the wall by it? ususally the wood frame of the window gets wet and mold will begin to grow. If you were to try to wipe it off the surface mold spores would come off but the wood would still be discolored from the mold that has bitten into the wood, the only way to remove it is to scrape out that wood or replace it all together. If you can just wipe the black stuff off after it has been there for a long time. Is is probably not mold. Never wipe mold away. Kill it first then wipe it clean. Every time you wipe mold beofre it has been killed you release millions of mold spores into the air and then breathe them into your lungs. In one breath in a clean home or outside (no where near mold) you inhale something like 60,000 mold spores (I think that is the right number..it has beena while since my last class). Sorry that was long but I am a mold remediation specialist. I get black sooty looking marks on the ceiling above my vents if I have not changed the filter on my furnace. It says right onthe label once a month or every 30 days the furnace is running. I also smoke. I am sure that is why the air in my house is yucky. This happened even before I smoked in the house, but know that I do it is worse, faster and darker. edited to say: try an air purifier that should help. also if your air intake is blocked i.e. boxes stacked up in front of it, move the stuff make sure your intake is free and clear.