The SS used by various gun makers contains enough iron that it WILL rust, if in nasty conditions long enough.
The original "hard service" finish for firearms was nickel. It was a tad on soft side but much more rust resistant than blue. Often seen up into the 1960's on S&W pistols used in law enforcement.
Hard chrome (HC) imparts a RC70 coat to a steel gun. It is electrically applied in a thin coat. It is a hard service coating that resists corrosion. It will mirror the metal finish underneath it. If a surface below is highly polished, that is how it appears. If the surface is matte, then that's how it looks. It takes lots of skilled labor to get the highly polished look; read that as expensive. HC will undesirably build up on sharp surfaces. Skilled labor required to reassemble a gun post HC. HC has a slightly blue cast to it.
Electroless nickel (EN) is about RC50. More rust resistant than HC, so often used for hard service service on/near to water. Some think EN has a goldish color cast to it.
A blued gun can be made more rust resistant by applying a good coat of non-abrasive paste wax. It was often done to blue LE guns pre-HC and EN days. For pistols often carried, reapplication of the wax was required frequently if in rough service.