The tip-off on that gun is not only the color,, but also the rounded edges that should be sharp, like the barrel angles. Those were always sharp and even. If they are not, a refinishing is likely. The only way to know what to look for is to look at guns, lots of them. That is why I strongly suggest that you hold onto your money until you go to some gun shows and LOOK AT GUNS. Some dealers will even let you photograph guns if you want (always ask!!). Look at colors. Look at cased sets, as those are more likely to have the original finish. Examine guns closely (again always ask before picking up a gun and NEVER cock or work an antique gun) to get a "feel" for what originals look like.
And buy books. Good books are expensive, but well worth it, not only in themselves for good reading, but because they provide information. It also helps to know history. There is an old joke about someone paying a high price for the gun Grant carried at Gettysburg. Anyone with a knowledge of history would know that Grant was not at that battle.
You seem to not be concerned much about condition. But in the field of antique guns, that is the main determinant of value. One 1851 Navy might cost $5000 and another $500 - the difference is condition. And if I don't know what to look for I might pay $5000 for the $500 gun, not a very wise move.
(Interesting about Lugers - ones made prior to the mid-1930's are rust blued, a "soft" looking blue. Ones made after that are hot tank blued. Again, knowing the difference is important; a 1910 Luger that has been tank blued is not original and is worth a lot less than one with its original finish.)
Yes, you can use cold blue to "enhance" the original finish by darkening worn spots, masking scratches, etc. It is a common practice. Cold blue has a distinct odor, which is why you will sometimes see people at gun shows using a literal "sniff test" on a gun.
Jim