I think you may be right with it being dirty, i had a plumber friend who does a little electrical work come over and look at it and he rubbed it right off and the wire looks fine. He said with breakers it's nearly impossible for a wire to over heat and burn, you will always trip the breaker...
The sheath looks fine (other than the burn mark), and I'm replacing the wire anyways because I'm moving the switch to a different wall and it needs to be longer. I just didn't know if there was a bigger problem somewhere else.
Thanks for the reply.
We are in the process of renovating our kitchen. Our first step was to remove one of the walls. After I ripped down all the drywall and exposed the wiring I noticed a small burn mark on the wire sheath where the wiring comes through the stud at the ceiling from the attic. The wire is for the...