The best instructions I have seen for rust bluing are in Dunlap's "Gunsmithing" book starting on page 388, called "The Niedner Cold Blue".
Beautiful, tough finish. No Mercuric Chloride. Seems to work on all steels: mild steel, 4140, heat treated 4340, even casehardened steel.
Granted some react faster than others. Follow instructions or it will etch the steel.
Chuck