After a close inspection, following its purchase, I thought to myself that at one time the entire coin had been coated with the black "goo" or "crust" as you call it, and the previous owner had tried removing it using unknown methods, with limited success. I did soak it in Acetone for a day with no improvement, followed by a soaking in xylene, also for a day, again with no improvement. That is why I was toying with using a dip. I really like the coin. Good detail, regardless of the surface. Since I didn't spend a bloody fortune maybe I will experiment and see what happens.
I've never seen dip (by which I mean the thiourea-based toning removers) affect black crust. If you want to experiment, get a toothpick and put a drop of dip by RIC on the reverse. You'll probably see nothing happen and smell no sulfur. If you want to continue experimenting, get a rose thorn or cactus spine and see if you can flake off any of the crust in the same area.
Thank you for the advice. We don't have cactus or rose bushes so will have to find something similar, possibly a blackberry thorn. To be continued.