A good question. The answer is, "virtually impossible." The technical, scientific reason is that rust bluing, found on many Mausers, or hot bluing on many of the later ones is done by a different process than cold bluing. For example, hot caustic bluing is done with alkali salts, and cold bluing is usually an acid. In chemistry class they'll teach you that the two, acid and alkali, are opposites and don't coexist well. If you put a drop of cold blue on a hot blued gun you will get a visible, differently colored ring where the two meet. The only way to get a flawless blue job, where everything looks the same, is to strip off the old finish down to the bare metal and then refinish with one of the bluing methods; or, as is now days becoming more and more common, one of the spray on finishes. Probably, the practical thing to do in your situation is to spot cold blue.
The purpose of any of these methods is to protect the metal from rusting. In actual fact, bluing processes are rusting processes that produce a blue or black oxide coating that is more pleasing to the eye than is brown or red rust. However, once some of the bluing is scraped off from a given spot it is unprotected and prone to rust red. Prolonged exposure to droplets of water or high humidity without the protection of some sort of protective coating like an oil or wax will cause the existing blue to begin to rust even without an imperfection in the surface.
But, you probably don't want to completely refinish the rifle. I would suggest buying the smallest possible bottle of Oxpho Blue from Brownell's and following the directions. A little prep with some rubbing alcohol and a Q-tip, apply some with another Q-tip and rub it a little, wipe it off with an oily cloth. Very easy to use with a minimum of preparation, and usually more difficult to see the repaired area than, for example, Birchwood Casey cold blue or 44.40 Blue where you're almost certain to see a ring. However, the pits will remain under the blue and be visible. Pits result beneath the rust because the metal has been oxidized into the form of rust and now is gone. Unless one is careful to get all the rust out of the bottom of the pits, given the opportunity they will continue to rust. The only way to get rid of the pits is to polish, sand, or file them out. That brings you back to the complete refinish job again. You can try filling the pits with various substances, but it is always visible. That is one reason that the spray on, bake on, finishes have become popular, because the substance fills and conceals the pits in most cases. It is also true that almost all of the modern finishes are more durable and rust resistant than is bluing.
A visual example here, which I've posted before on other threads. Honest, fellows, I know this isn't a Mauser rifle! I have a large accumulation of firearms acquired over about 62 years of active gun fanaticism. Among them is a small assortment of Star Model B pistols. One I purchased over the internet out of Kansas from a junk shop. It was rusty throughout, and especially so under the grips. I detail stripped it and cleaned it, started carrying it under my coat in the winter, and in short order all the rust came back. Gave it the same treatment, and the rust returned again. This is because previously rusty items like to rust some more, but also probably due in part to my warm body and the cold outside air creating condensation. One of the things that irritated me the most, and was the most difficult to clean, were the grip screws that seemed to show rust almost daily. I gave up and decided to paint it with AlumaHyde II, a Brownell's product in a rattle can. The pits were filled up and sealed against further rust by the paint. I replaced the ever-rusty grip screws with stainless screws purchased on e-bay. My personal taste prefers a nicely blued pistol as shown in the first photo, but the paint job in the second photo is now about 7 years old, and has proved very practical.
Somehow this Llama snuck in......?