The Pilot is normal , but it wastes gas. Non sealing doors are the
norm, a waste of energy up the flue. I fitted a foamboard plug.
The black is mold I bet , you have studs on an exterior basement wall
finished in drywall , right. Is it old ? Many basements seep moisture
and never should be finished. If you remove a piece of drywall I bet it
is all black. Do they have a dehumidifier down there, do you smell a
musty smell. I bet If AC is left off and no dehumidifier you will smell
Soot has an afinity for cooler areas ? Thats a good line of creative
bull shit. The wall leaks or seeps moisture, that is it.