Copper-nickel coins that are black are corroded. If you like the look, you should pay deep discounts for these coins because you will get paid a lot less for them when you sell. If you look at the surfaces, you will see that they are not smooth, but a bit rough. Nicely toned nickels are smooth under color. I know “crusty is cool,” but corroded isn’t.
I believe that @paddyman98 has a Black Beauty that he has shared before. Maybe he can repost the photo so you can see the difference. Your coin is just environmental damage, IMHO.
@paddyman98 let’s see it or are you sleeping on that bed full of your wonderful coins. It’s the one by your toe