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Rust is the term for corrosion of iron alloys, and is caused by moisture, an oxidant and a susceptible metal. Bluing provides only limited rust protection, being itself a special 'rust', Fe3O4 magnetite black oxide. Bluing needs an oil of some sort to exclude the oxygen.
Modern weapons surface treatment do a better job of preventing rust.
I had a spot of right red rust appear on the takedown lever of my H&K and they promptly replaced it.
Even stainless steel can show rust if the surface iron is not polished off. A test that I did on reactor plant parts had them wrapped in seawater soaked cloth for 24 hours, to be rejected at any hint of red staining of the cloth.