Found this as at least a partial answer:

Worn-out rings or injectors, ....poor fuel spray patterns and incorrect air-fuel ratios are major causes of soot formation. A faulty fuel nozzle may spray more fuel than desired, increasing the fuel-to-air ratio and causing incomplete combustion and soot accumulation, or the air filter may become clogged, decreasing air supply and increasing the fuel-air ratio.

And I guess raises another question: if soot production were becoming significantly worse due to one of these factors, would there be noticeable symptoms other than the soot itself?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife