A magic eraser soaked in z cleaner will wipe it right off. The flapper doesn’t help either. Increasing the exhaust gas velocity will get it into the slip stream and away from the low pressure pocket at the transom. I battled this and soundown designed a custom exhaust from the turbo to the outlet at the transom. My muffler is huge and slows the exhaust down to the point that it rises as it exits. The solution was to add a reducing tip from 6” to 5” to increase the velocity. It works great and the back pressure is still well within spec. Soap is a lot cheaper than what I went through but if you want to stop the soot completely get the exhaust into the slip stream. Good luck!