I do some charcoal blueing in one of the classes that I teach and it isn't too difficult with the right materials and equipment; I do find it a bit too shiny for some applications however. It is not a good choice for heat treated parts since, at least the way I do it, the temperatures involved approach 900 degrees. Here is a somewhat bad photo of a cylinder I did last summer, the surface is actually very deep and consistant, something the photo doesn't show. The spots showing in the photo aren't really there.
