Found this one in my stash of coins. After all I have read on the coin, I am still confused about it. It has quite a bit of dirt on it and was nervous about trying to it get off. What do you all think?
Looks like the discoloration is from environment as the alloy used for nickels and the outer clad layers on dimes, quarters, and halves are susceptible to environmental damage also resulting in a darker appearance. Unless you are getting them from mint bags or rolls it would be hard to know exactly what its been exposed to during its circulation and would no longer have some of the mint luster.
Black beauty nickels seem to be the result of improper annealment, the process by which planchets are heated to prepare them for striking. It seems these nickels were left in the furnace a bit too long, producing a darker color that can range from gunmetal gray to jet black. Some also think they may have an improper alloy mix. Improper annealment can be found in all denominations actually, but 1959 nickels are particularly known for this error.
Also Denver coins apparently are not usually found with improper alloy mix.