Many years ago when onthe farm & we butchered our own hogs we used saltpeter (Potassium Nitrate) in a recipe we had for Sugar Cured Hams. Chille was at least at some point in time a major supplier of Sodium Nitrate so it has often been referred to as Chile Saltpeter.
Any of the three can be used in Hot Salt bluing & are sometimes mixed. I have no idea though as to whether either of the alternates are compatible with Niter Bluing.

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