It looks like a piece of gum to me.....I don't see any Blakesly effect on the opposite side...have you tried to peal it off?
It looks like a piece of melted plastic or wax. FYI, cuds are not associated with the Blakesley effect.
Then is it the reverse of the coin that will have no details because of the cud? I thought that the excess material going into the cud caused the details for the coin to be lost in that area. Oops, answered my own question I think.
I thought it was gum when I first found it, but its a blob of metal and I have tried peeling it off with my nail but it is stuck on there. The reverse strike isn't weak. Could someone have added this?