I suspect that the black stuff is a complex of silver / copper oxides and sulfides. Shallow corrosion scarring may be underneath. Remember, this coin was probably recovered from soil contact burial, and most probably had a primary cleaning already. That would explain it's current appearance.
Try a soaking in sealed small container of acetone first, but I don't think that will do much good. To remove, it will require physical removal with a Q-tip. Patience needed.