A couple of comments on rust and bluing: Both are forms of iron oxide, but whether the oxide is blue or brown depends on the temperature at which it forms, from what I read. If it forms at room temperature, it'll be reddish brown. If it forms at higher temps (not far from boiling the solution, IIRC), it will be blue to black, which is why bluing is done in a bath of heated solution. These days, there are some very good compounds which can produce a true oxided surface chemically without the investment of all the traditional bluing equipment. One of those is Van's Instant Gun Blue - which is not actually blue. It imparts a black-ish finish which deepens with continued stroking with an applicator wet with Van's. I use it whenever I do something which removes the finish on a part like repairing boogered-up screw heads, scrapes and hand or holster wear, fitting dovetails on sights, etc. The surface must be entirely oil-free before you apply it, but it will oxidize the steel smoothly and evenly, darkening more with each stroke and is easy to blend well with a blue-black existing finish. Once the rust is removed, an area of minor surface rust should be easy to make virtually invisible. Be sure not to change the texture of the surface much, as this will change how the finish turns out, no matter what method is used.
There is/was a similar product on the market called Insta-Black which is intended for oxiding tools with a very black finish. It would work well on guns as well I would think, but tends to be more aggressive. It would be good on sights, though.
There are some to stay away from, like a "cold blue" pen I saw once, which worked just like a magic marker and did the same thing, essentially just coloring the part blue. No durability there. One hunt and it had to be re-applied. Cheesy.