I am not an expert on cleaning coins, but based on chemical principles. the best substance for removing "tar" without reacting with the coin's surface would likely be an organic solvent like mineral spirits, or a commercial product like Goo Gone which is designed to remove grease... These would have a chance to dissolve the tar.
If your coin is a proof, you'll need to take extreme care not to damage the finish (I am guessing most collectors would say to leave the coin alone because of the risk of causing more harm than good.).