Open that little door shown in the last pic. You may not like what you see.
If there is a coating of soot lining the wall of the pipe,that means your boiler needs pro servicing. Most of the soot you are seeing is probably coming out of there.
Probably a new nozzle,filter,and a tuneup,at the least. Don't do it yourself. It most likely by this time needs to be wire brushed inside and vacuumed out. You don't want to ruin your shopvac doing that.
The service guys have all the right tools for it. Don't fool around procrastinating. That can be very dangerous if it gets worse.
It must get boring preaching to the choir.