Not for sure on this. I thought it only happened in the 50's with Nickels. Sure does look like the 1959 I have. ( THERE IS LIGHT DISCOLORATION ABOVE THE MONTICELLO. LOOKS A LITTLE BROWN)
It seems that the improperly annealed planchet issue for nickels is not just confined to 1950s nickels. https://www.error-ref.com/improper-annealing/
I've read that improperly annealed coins can happen in any year, but I can't tell from the photo. Maybe send it to Fred or Joe and see what they say with the coin in hand.
see on the top nickel, the "luster line" reflection of light, that will swirl around the coin, this effect should still be present on the darker nickel if the darkening occurred before the strike (like from annealing error), the strike luster remains. if it came after the strike, the color/toning certainly will impair the strike luster, just as circulation wear will impair the strike luster. It's not fool proof, just about impossible for me to tell if it's annealing or ED once the coin has circulated enough and the strike luster is gone. in fact it's a pretty useless diagnostic if it's a circulated coin that has worn. I would say this edge view though, shows signs of corrosion on the edge to me, circled in red. This would likely indicate it's going to be ED. Just my unprofessional opinion, I think this is ED and reasoning as to why I think it is. there's always a chance of a coin here or there getting hung up in the annealing step and cooking longer than it should. However I'm not a proponent of the theory that in 1959 a batch was left too long and that's how it happened. I'm believing the annealing oven needed to be refurbished at the end of 1958 beginning of 1959 runs in Philly, and was leaking air into the furnace, and this is how the black beauty nickles came to be, perhaps they patched it up, and worked with it the best they could until they refurbished it. No way to prove it, but there were enough of them in 58 and 59 to get noticed, so I dont' think it was a hang up of just a couple, but I don't think it was a entire run of them either, just a annealing oven leaking more air into a section, turning the coins near the leak a dark color.
Give a nickel to a kid if he doesn't spend it that is what it will look like when he gets through with it. Now you will have the opportunity to spend it. You already have the correct answer, so I am just offering a bit of humor. Thanks, and good luck.
