Even molten lead can be used to color steel. As long as the proper temp can be reached and held for the desired color, it matters little what the substance is.
Though lead isn't a good one to use for higher temps,,either the Brownells salts or any form of sodium nitrate or potassium nitrate will do.
Take them to 850F and the color will be a deep blue/black.
The 650-700F range gives a sapphire blue color, often called 'spring blue' color as it is the color and temp range that springs are tempered at. Which is most likely the real use for the process,,drawing temper.
Carbona blue is a completely different process done in an revolving oven with bone or wood char and in the patented process,,Carbona oil.
True charcoal blueing different again. What they all have in common is that they place the parts to be blued out of contact with the free oxygen of the atmosphere. Some variations of each.
Just heating a part in the air and getting heat change temper colors is just that. But on small parts it does a nice job if you can control it well and is fast and easy.
This is what I'd refer to as 'fire bluing'.
Some English WW2 military parts were 'Blackened' by dipping in oil and burning it off in some manner, perhaps in ovens to control the temp. It gave a fairly decent finish against abrasion, but it's best quality was that it was fast and easy to do I suspect.
I'd guess it or something similar was done in the past too.
