Unburnt petrol making soot in the exhaust. Its a sign of something could be improved. Air cleaner clogged? Air leak? Timing? Bad fuel? Bad plugs? Dead cat? Burning oil? etc, etc,
Our cars have lean-burn engines, and the exhaust should be bare metal maybe with a hint of pale brown.
Fifteen year old original exhaust on MX-5 NC occasional heavy right foot.
Seven year old original exhaust on Mazda3 skyactive engine, even leaner burn (most of the time)
But then my 1996 Vectra exhaust looked like the Mazda3 exhaust all its 20 year life too.