My tried and tested heat bluing of gun pins (screws) giving a good deep lasting blue colour is.
Firstly heat the screw to the correct bluing temperature 570F or the colour you want by eye, slowly to increase the oxide thickness. To halt the colouring process drop the screw in oil. This is where we modern folks have to substitute something for the Whale oil that gave the best colour in days gone by.
Over the years I have found three substitutes that work extremely well, though all require some heating to make them a liquid. My preferred oil is "Lard" second is "butter" the final one is "Tallow" though I do not like the smell Tallow gives off when the hot metal is dropped in.
As you can see they are all animal derived oils leaving Whales to get on with their lives.
