I just don’t understand you folks over there and this rust black or blue thing there is positively no secret about the whole process. Rust colouring is not what we would call a straight forward industrial process it is more an artisan process that consists of trial and error producing skill in the person who does it, so it makes no difference on what side of the pond you where born only how much practice and effort you are prepared to put in to perfecting the process for your self, and if there is a secret that is it.
The formula secrets went out of the window when R.H. Angier a country man of yours published his work “Firearms Bluing & Browning” in 1936 incidentally still in print today and very informative it is to.
So if you want to try your luck purchase a copy it is not expensive locate a good laboratory chemical supplier, but one word of advice keep well away from “Mercuric Chloride” until you have gained some experience or the only thing that will be turning blue/black will be you after all life is extinct.
