I'm not into originality Cratebuster but it looks as though some of the old brassware was just left to darken naturally. Others have been chemically blacked Maybe a chemical brass black could be used.
There is a hard, hot applied wax stuff we use to highlight the drum markings but reasonably, an kiddies playground wax crayon could be used.
When they came through our workshops, any notion of originality was forgotten about and they went through 'the works'. This 'works' didn't feature blueing or polishing brass etc etc. They just followed the rifle format!