If you are just doing a few lock bolts, top jaw screws, etc., use a propane torch and a pair of needle nose pliers or tongs to hold the bolts.
Carefully buff the surfaces to a mirror finish. De-grease them thoroughly with lacquer thinner, acetone, etc. Dry completely. Heat the shank of the bolt SLOWLY with a small flame. The temper colors with gravitate away from the heat source in both directions. Watch the colors move towards the head and tip.
You will see the colors develop starting with a staw color, then brown, purple and just after purple, you will get your temper blue. Immmediately quench in warm water. (You can quench the tip or the head and continue heating the rest if one end is blue and the other is not.) Check the color again to make sure you have what you want. If you heat too fast or too hot you either will not be able to control the color or you will go past the temper blue to a "sky blue". If that happens, re-polish and start over.
When everything is the right color, dip in oil or grease preferably while the part is still warm. Wipe off he excess and you should have it.
One word of caution: Sometimes slight variations in the steel alloy will affect the end color. (Some will blue bright and others will blue a different shade.) If you can try and get bolts from the same manufacturing lot.
Good luck
