years ago I rust-blued a half a dozen handguns using the old fashioned Niedner-blue as given in the Angier book   starting with various acids and dissolving small finishing nails in the solution  with many applications boiling on the stovetop and "carding" I plugged the well oiled bores with corks and a "periscope" at the muzzle.  I found that that blue gave the hardest most durable deep blue-black I have ever used.  I used in one the frame and slide of a 1911 Colt----maybe close to 40 years ago.  In spite of daily use for years and less than optimum storage at times it has never shown much wear other than some scratches.    I have a project in the works that will require a good rust blue and I'm thinking about revisiting that old formula and building  rust-box.-----assuming one can still buy the acids anymore.
If not are any of the commercial rust blues anything close to the Niedner Blue?

I found this product available for the US folks

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A pretty good deal for $35 other than its a bit strong and there is much more than you need.
Here in canada I have to take a 2.5L Carboy for $450. $45 worth of acid and $400 worth of CYA