While all of the issues noted in previous posts are valid. All diesels produce soot. Diesel does not work at 100% efficiency. That couple of points of inefficiency produces particulates in the exhaust. The air flow around the exhaust may suffer from the station wagon effect and stain part of the stern. In my own boat there is no visible smoke but there is a 20" circle of very light staining over 2' away from the exhaust outlet after a long days run. Being that far from the exhaust can only be accounted for by the air flow around my fantail stern. I have a plan to move the exhaust outlet next summer when she is hauled to do the bottom.
I had the same staining on two previous boats that also showed no visible smoke in the exhaust. I moved the exhaust outlet from the transom to the side on one boat and put a 2' FRP extension on the other and never saw another stain.
Aphorisms to live by .....
If you can live with the consequences, go for it - wg
Y'am what I y'am an' thats' all that y'am - Popeye