I’ve seen this old extreme tarnish. Probably started as a reaction from sweat or something coming from person(s) wearing it. I’ve seen some heavy tarnish stain metals.
Maybe some others can give advice of how to go about cleaning or possibly professional can help.
Tarnish that black can be extremely difficult to get off - it will take patience and gentle compounds, and even at that there are no guarantees that it will completely come off.
There’s a difference between “patina” and letting it go completely black, which can affect the silver permanently.