Note that the "Hot" "Salt" bluing attacks the solder joint through chemical reaction. It operates normally no higher than 300°F often around 285° which is not hot enough to melt the joint. Nomenclature is often confusing. We sometimes here of hot rust bluing & cold rust bluing. Both are normally subjected to boiling water, but at different times in the process.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra