Have you had the coin since 1993? If not, it will almost be impossible to say it is an annealing error. I also doubt the zinc small cent needs annealing. If they do go through annealing then it is probably at a fairly low heat for a short time. They definitely go through a wash of some sort.
I do not believe that copper plated zinc coins go through an annealing process. I say this because zinc does not not have the "Strike Hardening" property that copper has.
It is normal circulation toning. A lot of Copper Plated Cents get Environmental Toning. Millions of them.