Iron rust occurs as either ferrus oxide (red rust), Fe2O3 (a molecule made up of two atoms of iron and three atoms of oxygen), or ferric oxide (black rust), Fe O2 (a molecule made of one atom of iron combined with two atoms of oxygen). Since more oxygen is required for black rust than for red rust, the red will occur more frequently in nature than will the black. Red can be converted to black by boiling in water; the oxygen disolved in the water will combine with the red rust readily at boiling temperature and convert it to black. SOoooo, if you have black anywhere on the barrel, it is a good bet that it was originally black as the black was replaced by red as the original black wore off. Increasing carbon content of steel reduces its rate of rusting (red or black). The varying carbon content of the two steels will ctause differing rust rates and allows the damascus andtwist paterns to be developed.
