One of my sister's and I have a patch of hair that is different color than our natural hair color. Hers is a blond patch with dark brown hair. Mine is black with dark brown hair. We've had it for as long as we remember; our Mom said it was a birthmark. I looked up congenital pigmented nevus and that doesn't describe at all what we have.
On another site, DrewzGirl had this to say about the coloration: "No, its not a birthmark. I am a cosmetologist and know for a fact that birthmarks have nothing to do with the type of pigment in the hair. Eumelanin, the pigment used in brown hair was just genetically given to him at birth, and the little patch of blonde was probably just a recessive gene that slipped into his hair follicles where he has the blonde patch. Its completely normal and since hes just 8, his hair color very well may change over the next few years (i.e. pre-pubescent) as this is common."
This still sounds like a birthmark to me and gives a good reason for it. Now that my sister and I are over 55, our patches are hidden by our dead hair. :(