How about a rust blue?
Or, a simple heat blue?
I have not charcoal blued any parts as yet, but I have to wonder about scale forming in the bore of, an otherwise, finished barrel. If the "blue" on the exterior is formed by scaling, then logic suggests that the same scale has to form in the bore. I don't know what effect this scale in the bore would have on accuracy, but it couldn't be good.
Just kinda thinkin'...typing out loud, so to speak.
J.D.