[DonVoigt]

Dave,
I think you meant to say "rust " bluing (not salt bluing) = blue tinge.

Salt blue(hot) and hot blue(black oxide) are the same process using melted "salts" - mostly NaOH and Nitrates. It is done at a temperature well above that of boiling water, due to the higher boiling point of the melted salts.

Rust blue uses a relatively dilute solution of nitric acid with "additives" and requires development of a rust layer which is carded away and the solution re-applied, perhaps 4 or 6 or even more times to achieve the desired finish.