Somewhere laying around I have a paper that was done by a researcher who determined that browning was not common in the 18th century. Most guns today in museums, even Trapdoor Springfields from the late 19th century that we know for a fact were blued, have acquired a brown color through natural oxidation of the bluing. The author's contention is that the vast majority of rifles were sold with the barrels and locks in the white, and that a few customers opted at extra cost for fire-bluing of the entire gun. This is still done today by the bolder gunmakers, and it means laying your 200 dollar barrel into a fire and letting the fire do the bluing work for you.