I seem to have fixed this on my machine. On the front of the inside of the machine there are some deep holes molded into the front cover, these had filled up with dark lint over time. It seems that once the lint builds up to the surface of the molding it can rub against the clothes and cause the marks. I got a vacuum cleaner with a brush fitting and a little stick and cleaned the holes out, I got heaps of lint out. So far - no more black marks.
Andrew, December 2020