Recently my husband and I have noticed thick black dust in our air filter and on the intake side of our fans we have in our apartment. It almost looks sooty. We don't burn anything or smoke in our apartment. We don't open the windows and we keep the A/C going at about 77 degrees. This has happened in the past month. The apartments we live in are about 15 years old, and are very well taken care of. There is no known mold problems in the apartment (we are both horribly allergic) Any ideas what this could be? Its getting to the point I have to replace air filters twice a month and they are still clogged with this thick black dust. (thank goodness they provide them for free!)

YOu need to check to see if the chimney is venting properly and if you have a fire place its not down drafting.
If you rub your fingers on this dirt does it smear like a grease or is it just dust like?
If you were allergic to what ever this was you would have had signs long before this occured so I concure with you that its not mold.

Poorly vented furnaces and hot water heaters can cause a black soot to occur.

What about your dryer vent. Have the clothes been taking longer then usual to dry? That to can cause dust in the air.

What about the intake grills in the house? Are they collecting this dirt on the fins? If not the issue is within the duct system going to the filter. Perhaps there is a broken duct within the walls that is drawing dust from another unit.

Regardless of the issue. My bet its something to do with the venting of the equipment.

Be sure your CO tester is working properly. You can always call the local gas supplier in the area for a free evaluation for this issue.

We dont have a chimney, dryer or furnace. Only a gas stove/over. We don't even use the oven and we keep the stove top free of any debris so no smoking occurs.

Your heating with a heat pump then I gather.
Electical hot water.

I have no clue then what is causing this problem without seeing the home.

You say that this just started? Home is fairly new?

Do you have a central return next to the air handler for the AC? Or does the return get piped from another location to where the system is located? Single return vent or many in all rooms or levels?

Is the filter located at the blower or at the grill location?

Only things I can think of is to check the motor mounts on the blower fan, a drive belt for the fan if it has one, or finally a door gasket on the unit. If any of those rubber parts are disintegrating, they could cause some black dust.