I don't do hot stuff any more, rust blue everything now. In the past I used the below. It's from Roy Dunlpap's book "Gunsmithing".

Lye 2 parts (or sodium hydroxide) Lye is usually 94% NaOH
Ammonium nitrate 1 part (ammonium nitrate fertilizer 33%)
Water see below

Solution: 5lbs lye
2 1/2 lbs ammonium nitrate,
per gallon of water

Work temp 285-295 F. Rinse in warm/hot water.

Gives a nice dark blue/black black/blue depending on the steel.