peraspera,
I wonder if what you are seeing in the form of color change is really carbon (there just isn't that much carbon in Ni200) but most metals tend to change color when heated. This is even done with some metals to give it a unique look.
Also if it was some oxides of some type you would definitely see it on the paper towel.
Whatever is on the surface is matte, not shiny, and for sure does not wipe off. I was just guessing that it is carbon as the data sheet indicates carbon will leach out with high temps. The only thing health-wise that even comes to mind is that the matte surface might not be as chemically resistant as the shiny one but I haven't a clue if that would be true.
From a build perspective the matte stuff might have some impact if someone is doing touching coils with some parts ending up shiny and others matte. Might it cause the coils to short unevenly to one another causing the DNA40 chip to have a snit fit??? Haven't a clue about that either.
