Starbright Black Streak Remover
I had black streaks running down the hull/topsides from a nearby fire, similar to the black diesel engine exhaust stain I sometimes get on my transom. After trying boat soap, simple green and similar cleaners with very limited success after lots of elbow grease, someone suggested Starbright Black Streak Remover.
It worked like a charm--spray on, wait a minute and wipe off! It only cost me about $8.00 for a spray bottle that lasted all season.
I have since then seen Black Streak Remover made by other companies, including in the RV section of department stores, but I haven't wanted to risk getting less than the great results of the Starbright stuff.
I have no connection with any of the products mentioned.