If you've seen more than a handful of fine Swiss watch movements, chances are you've come across one fitted with screws which were colored blue. This is more for decoration than function.
Although the bluing can be done by chemicals, the proper method is more time intensive and costly.
To blue a screw head you first have to clean it up, polish it, and then remove any oily or greasy deposits from the surface. The more the screw is polished prior to heat application, the richer the blue hue will be.
When steel is heated, it will go through a number of color changes eventually changing to a rich blue. This is generally an automated process with complex machinery but the same effect can be achieved with a pair of snipe-nose pliers held against flame. Once the color is achieved, it needs to be immediately doused in water, dried, and rinsed in solvent.