I do the camo color first, then black paint inside of the brake finally pencil to highlight, like all have said. Usually a muzzle brake is given a cleaning each time the tube is punched, so there'll usually be some paint showing through also... especially on the 'front' edges of it.
Maybe Redleg or the Major can verify this, but I don't recall ever seeing black on the front of a muzzle brake. The paint will be darkened from heat and cleaning (getting carbon rubbed into it) at times, but the carbon (nice 'n' black) tends to be inside of the brake. I have seen MB's scoured down to the metal, and one that was silver in color on an M48... However, as far as charring goes, I think the flash is so brief that the brake doesn't really heat up all that much (It's also kind of a natural radiator)... so, to my knowledge, which isn't always right, the black on the brake is carbon, not charring.
Ron.