problem is with PMD .. it could be literally a billion things. leave it up to one's imagination of what ppl can do to things. Does it look like solder ??
To feel it it seems like part of the nickel. I'm just a rookie here but I work with my hands a lot and know a great deal about iron , steel and metal work. Nothing added in my opinion.
Safe bet it is some sort of damage. Based on the wear on that nickel, it has certainly passed through many collector's hands over the last 60 years who would have removed it from circulation if it was an error of any particular note.
What year is it may be some errors Doubled dies and RPMs Also looks to be cleaned never clean your coins
@Weyandt That is not a Black Beauty. Yours is an old dirty Nickel http://www.error-ref.com/improper-annealing/ These are Improperly Annealed Nickels from my collection -