Pure silver will tarnish black due to the sulphur in the atmosphere. Copper has a slightly higher affinity to sulphur than silver so throw a copper round in the box where you keep the silver to help stop the tarnishing.
Coin looks like a RAM 1988 $1 coin. My 1990s Perth Mint kookaburra proof coins have a similar problem.
What a shame. That special piece is reduced to silver spot price. I understand why so many are coming out with antique finished (Poland Mint)....because those black stains are an eye sore
Chemicals and elements in the air cause corrosion to virtually every surface. Now the rate of corrosion will vary greatly on a number of factors including metal composition, the composition of chemicals and elements that are in or comprise the air, and even temperature (as far as I understand). Pitting (at least that's what it looks like) is what you see on the rim of the coin in your picture. Before excessive corrosion begins, there are things you can do to mitigate the likelihood of rapid corrosion to a coin which includes common sense things like don't use PVC plastic to contain the coin, do use a more inert plastic holder, try to minimize air contact, etc, etc, etc. Corrosion cannot be undone once it occurs as far as I'm aware.
It looks like a silver proof $2 coin, I would say there was possibly some sort of contaminent on the inside of the capsule. Are you the original owner? If not it is possible the original owner has taken it out of its capsule and the coin was contaminated. Does it look like tarnish or some sort of organic matter?
