With no picture to view, and you said you cleaned with no success, I would say they are Tarnish/toning; these dark or black spots are usually silver sulfide.
Once they are formed, they become part of the surface - not removable without damaging the coin's luster. I would leave it alone.
Once they are formed, they become part of the surface - not removable without damaging the coin's luster. I would leave it alone.


















