"mm" wrote

I can imagine that sooty air escapes the fire chamber, permeates the
house and deposits itself everywhere, but that isn't what happened.
It's only on the sheetrock nails in most of the rooms where the heat
was on, on the walls where the vents are, and by the mail slot (which
I have now attempted to put weatherstripping in.) which is not too far
from a heating vent.

If you are getting soot out of the vents, then you probably have a cracked
heat exchanger instead of a cracked firebox (or course, a cracked firebox
can cause a cracked heat exchanger, but I digress).

Good possibility: when the chimney was stopped up, you were getting a lot of
back pressure. Depending on the design of your furnace cabinet, sometimes
the blower compartment doesn't have a tight seal and it cab suck fumes/soot
into it a blow it throughout your ducts.

Hey! It may be O.K. Just have it checked over real good (every year!) and
keep those batteries fresh in the CO detector...