I seen them like that. I seem to recall there’s even a chemical that will turn them black like that, but don’t hold me to it. At any rate, I don’t think that’s tarnish, I think something reacted on it.
The coins that @paddyman98 digs seem to primarily have a rust color to them. Coins that come out of the ground in my neck of the woods tend to be more like the half you are showing. Of course it could be anything but something has certainly reacted and turned that half.
The outside layers of the 40% silver halves are 80% silver, 20% Copper, so you can see toning/tarnish similar to a 90% silver coin. It looks like some type of environmental damage, but outside of a guess, there really isn't any way to determine the exact cause. Not really a big deal, because whatever happened, happened after the coin left the mint. It's ok to be curious about the causes of PMD, but don't get too worried if you get different answers on here. There are countless ways a coin can get damaged/tarnished after it left the mint.
the black is usually indicative of silver(I) sulfide (Ag2S). what would be called in the hobby, "terminal toning" of the silver.... silver oxidization. in fact someone could have done it on purpose even. Jewelers do it to silver, call it "oxidized silver" or "blackened silver" just speeding the process with chemicals to give it an antiqued look that wears in as some of the oxidization wears off. Could have been a test a jeweler was doing before doing it to their work piece. Not sure of the cause to get it to that point, the specific circumstances, or how long it took. Given enough time and the right conditions all silver coins can get to this point. I'm fairly certain just like silverware, it could be removed with a coin dip or any silver cleaner, however with toning to that extreme, it's certainly going to have damaged surfaces under it unless it hasn't been there long and was accelerated to give it that patina. it's still a 40%er though.
