Sal they are called improperly annealed. This is when the planchets spent too much time in the annealing oven.
I have specimens of improperly annealed coins of every denomination except cents and I always wondered why it doesn't happen to cents. I should have realized that it was the metal composition. Duh!
There is a post in this thread that says all burnished coins look burn't Err ref says that these can be light and brilliant like the buffalo in Sals post. 1967 Nickel is this a black beauty or just toned? | Coin Talk