It is a tough thing for many to understand until they can replicate the problem itself which is raw gas being pumped without it being burnt.
The only carbon left if from residuale combustion entering the exhaust system which ignites the raw gas.
Everyone has always kept to the mantra of
blue is oil
white is coolant
black is raw
But black is raw that a cat is trying to burn or an unfinished combustion cycle.
Had a example of the problem of raw gas ad white smoke but not coolant.It smelled of pure gas out of the exhaust.
Finally found a bad injector (stuck open) coupled with a faulty coil ,so it had little to no carbon to make it black.