I'm not about to guarantee it would work, but I have become a big fan of oxygen bleach. In the past couple weeks, I have been rebuilding a fence that was rotted. The fence was built to reduce road noise and it built next to a wall, so the two function as one. The prior property owners used the fron of the wall as the site for their compost pile, mostly from grass clippings. The pile is a big dark, dirty, stinky mess. I walk through it and pull myself up on the wall, and then need to work on the wall both sitting and kneeling on it. My clothes have been covered with some dark and stinky stuff. A couple scoops of oxygen bleach and hot water in the washing machine does a good job. Try it.