This soot is produced by that engines
Exhaust Gas Recirculation Valve
which should lead fumes to a second combustion, therefore kind of "cleaner" exhaust gases, longevity of particle filter extended.
But you buy all this with clogged valves, which are not cleaned anymore but changed from socalled mechanics, for a nice fee o'course. In case of my V6 VOLVO bill would be €1.600,- new valve + labour.
I did it in three hours because a lot of stuff has to be removed. € 0.00😀