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Carbon steel + blued finish = rust. Exactly why plating was done to firearms decades ago, first gold, then chrome, then electroless nickel, now nitriding and cerakoting.
Regardless of maker or process, a traditional deep blue on carbon steel will eventually rust. In the early days working firearms weren't blued at all - they were sold in the white and the owner expected them to rust, working in various oils to slow the process. By the time metallic cartridges became the norm, bluing was common, which did allow a modicum of protection compared to nothing at all. The surface was already converted to some degree and less likely to rust.
Note that even stainless will rust, it's called stain - less, not rust proof steel here in the US. Albeit the German name is rostfrie, which implies the quality.
IIRC scandium and titanium are for our purposes rust free, but that's an unfair comparison simply because there is no iron involved at all. And, neither does polymer. But if it's got Fe in it, it's gonna rust, and aluminum will corrode.
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