What is the process and how difficult is it to do quality blueing like you might get on an old Belgium Browning ? I rarely see the deep luster of the older guns and really know little to nothing about the process.
The bluing you are talking about is not typical hot tank bluing such as "Oxynate #7", it is actually rust bluing, "fume" bluing or so-called Belgian blue. Which are all similar processes.
Maybe we are not talking about the same thing...modern hot blue is very deep and almost black. What you describe, "on an old Belgian Browning" has more of a bluish look and also has a kind of silvery glow to it.
Metal prep is one thing and while it definitely contributes to the finished product, actually coloring the metal is another. As far as the coloring goes a good quality Belgian blue can be done with little more than a tank to boil water and the parts in, a very fine wire brush and the chemical. It is swabbed on and left to "rust", then the parts are boiled and the "rust" turns black. The part is dried and carded off with a fine wire brush. "Prep" as well as the type of steel determines how many times you have to repeat this to get the desired color.
This is a pretty simplified "in a nutshell" description and there is more to it, but in general this is it. Fume bluing is done in a big cabinet and the chemical goes on as a vapor. End result is the same, it's just a faster way to do a lot more parts.

This is a Lefever EE grade from 1905, notice the blue glow of the barrels.

This one is a Browning Superposed 20 gauge from about 1937. Same bluish glow...

This gun, a Winchester Model 12 Super Pigeon dates to 1961, notice the blacker, darker color of the hot bluing in use at the time.