The ingredient used in Brownells nitre blue or stump remover is potassium nitrate, also known as saltpeter. The stump remover isn't a real pure form, but it can be used. The melting point is 633.2 degrees F and it becomes a clear liquid when melted. Small parts and screws can be submerged in the molten bath to blue them. This is usually done by suspending them in the bath on a wire. As I recall, the parts are quickly quenched in water when they turn blue. It isn't the most durable blue there is, and it is fairly translucent, but it is attractive for certain small parts.
The process requires caution because molten saltpeter at over 600 degrees can cause some nasty burns. Also, it solidifies into a solid block upon cooling. Then when you heat it again to reuse it, the bottom will melt first, and it can erupt through the still hard surface and spray you with hot molten material. It is recommended to leave a threaded bolt in the bath while cooling, and then unscrew the bolt from the solidified material before reheating in order to have a vent going to the bottom to prevent those nasty eruptions.
Carbona blue was another heat blue process used by Colt, Smith&Wesson, and Luger. The parts were heated over a charcoal fire to around 600 degrees and variously tumbled into bone meal, leather scraps and other organic compounds, or wiped with pine tar oil or fish oil. This process supposedly added carbon to the surface of the steel in addition to coloring it. A high polish was necessary in order to get a rich deep blue. You can Google this to get many other variations on the process.
