During the Korean war in the early 50s Medium Red and Green period, the bright nickel plated parts and some brass plated parts including nuts and bolts, brush wheels etc. were supplied in black. The finish was continued later on the Army Multikit brackets, nuts and bolts. This finish was achieved by 'Brunofix', a proprietary metal finish with a secret formula, including a strong alkali and an oxidising agent to produce a black iron oxide finish. These chemicals, even if we had the formula, would be dangerous for home use.
Fortunately a nice semi-matt finish which preserves the surface is easily achieved at home.
A = rusty fishplate before any treatment
B = after tumbling in a tumble polisher with fine grit, and blueing in a reducing flame
C = finish after the process
Gas blowtorch or other burner capable of producing a blue flame
Pliers, forceps or other means of holding hot parts
Jar or tin of mineral oil deep enough to cover the parts
Heat the part in a reducing flame, to red heat. This is at the tip of the inner (pale blue) cone of a gas flame.
Immediately drop the part into the oil. Smoke will arise - do not inhale this!
When cooled, remove from the oil and wipe clean.