Maybe Due to V-Belt
The black "stuff"in the engine room coujld be V-belt dust. If it is, it is a really fine power. Look around the alternator pully - if it's worse there and in the neighborhood of the alternator that is probably what it is.The black stuff on the outside of the boat is a different issue and sounds exhaust related. Recommend a search of old posts in the archives for lots of discussions and you'll probably, no doubt, find some that fits your situation.