It looks kind of like corosion of some sort, oh yea it' a dime. I'm not worried about value I don't sell anything I find. Everything is legible (except for the date )
This sounds strange though it works well. Boil your coin (hard boil like an egg) for about 10 minutes in a solution of lemon juice and baking soda. You gotta keep an eye on this. As it begins to boil it will start to bubble over! Just use a pan with a handle, carry the pan over to the sink when it start to bubble over and blow the bubbles from the top of the pan then return it to the heat. Be sure to use a spoon to flip the coin a few times in the solution. It may take a few times to get the gunk off of the coin depending on how bad the black gunk is.