Place bluing salts on a china crucible or heat-proof tray. Heat from beneath until the salts are molten. Put fine wire onto any clean steel screws or hands to be blued and place them in the molten liquid until blue. Remove and allow to cool. The salts can be re-used again and again.
Gun Blue can be used but the result is normally too black to match the other blued parts.
If you blue items regularly consider using a variable temperature hot air gun. The steel must be clean and have no signs of discoloration. Keep an eye on the colour as it heats up. As the temperature increase the colour will change. Beyond blue is grey, and the only way to go back to blue is to clean the surface and start again from the beginning.
Repairing Your Own Clocks by Mervyn Passmore