References to photograph would be helpful. The smokebox end was black. Not graphite, black. As others have pointed out the silver color was for visibility. Not an issue on an AC's smokebox.
Silver is another color which can be easily formulated in a heat resistant paint.
It had a warmer tone, but was basically a black color. Shots of engines taken before the 30s show a dull finish on the smokebox which could have been graphite. As Arved says a heat resistant paint was needed on smokeboxes and fireboxes. The graphite finish provided that. As I understand it the graphite finish was linseed oil, plumbago and graphite mixed together.