Thermal bluing in a controlled oven is repeatable, efficient, and how most of the industry does it. We heat screws one at a time on a brass plate over a flame, watching for the exact moment the oxide layer turns the right violet-blue.
We ruin perhaps one screw in twenty this way. The nineteen that survive have a depth of color the oven never quite reaches — and a watchmaker who has personally watched every screw in your movement change color.