More important than that, PhillipP isn't talking about rust on bluing. He's talking about rust on color case hardened steel. Color case hardening isn't a bluing process per se. It is a carburizing process that case hardens the steel and results in some surface coloring, too. It is done by heating the steel to red heat in a closed container surrounded with bone charcoal or leather scrapes or other carbon sources, then quenching it. This hardens the steel surface a few thousandths in, and also porduces the uneven oxide that colors the steel. That oxide is very thin and doesn't offer much corrosion protection and wears off easily, though the hard steel underneath slides very easily against other steel and makes for very smooth feeling actions and triggers or other parts.
One of the characteristics of very hard steel is that it will rust, but doesn't pit as easily as softer steel. Pits are bad because the rust in them tends to migrate deeper into the metal. You can loosen the surface rust with any penetrating oil, such as Kroil or PB Blaster. Gunzilla, a vegetable oil based bore cleaner, is also very good at removing surface rust.
If you want to do a rigorous job, disassemble and degrease the rusted parts with denatured alcohol, then suspend them in boiling distilled water for 15 or 20 minutes. That will convert loose rust to magnetite (bluing). When you pull it out, pat it down with a towel and grab the barrel with an oven mit and shake it to get any loose water off. After it cools for 5 or 10 minutes, submerge it in WD-40 (get a gallon can from the auto supply) for two or three days. That will pull any remaining water out of the barrel threads and other nooks and crannies. When you pull it out, wipe it down to get rid of the loose black oxide on the surface. What remains has the advantage that it won't promote rust. That's the purpose of the exercise.
Finally, you do need to use a rust inhibitor on this metal whether you boil it or not. Birchwood Casey Sheath or LPS-1 will work. Spray the thing down with it to displace the WD-40 after boiling and wipe away the excess. For longer term storage use LPS-3, which is wax-like, or spray on Boeshield T9, which is also waxy.