Room dirt?

We have had our Enviro Empress FS in a finished basement for 3 or 4 heating seasons now.
I have noticed a coating of very fine black "dirt" on white doors we have in that room...also have noticed an accumulation of fine black "dirt" on the carpet where a door sits closed, as if the dirt was trapped under the door - the doors are very tight against the carpet, so I can understand particles in the air getting trapped in the carpet when trying to get under the doors.
I have washed the doors (painted) with Fantastic, and it takes 2 or 3 cleanings to get all the black off...it tends to smear with the first or second cleaning.

The walls in the room are taupe in color, so dirt would not be very noticeable on them.

Can anyone shed light on what is going on, or what is generating this "dirt"?

Is this fine ash, being blown by the convection fan?

Thanks.

Kenmore (household) canister vacuum (not HEPA)

Filter , bag or both?
There isn't much of a filter on the unit, items go right into a bag, and there is a small piece of mesh covering the opening in the bag chamber , that goes into the motor area (where the suction comes from.