Even if it were possible to get a very toxic material (nickel) to adhere to rusted steel, the temp of molten nickel (2651 F) would play hell with steel.
He may have meant the use of a high nickel welding rod, typically alloyed with other metals (boron, copper, silca, etc). However, if the metal in a filler weld is not the same as the base metal of the barrel, the bluing will NOT be the same- just changing steel alloys changes color of blue. Or refuses to be blued at all.
Bluing is a form of controlled rusting, forming back iron oxide. Nickel is not iron.
What we have here is... failure- to communicate.