I've read up on a few bluing formulas and most called for potassium nitrate, some used ammonium nitrate because of availability but the process had to be done outdoors due to ammonia fumes, any nitrate salt will do the job, and I'm sure sodium nitrate will work but potassium nitrate, being non hygroscopic and non fuming, is more stable in the formula compared to sodium/ammonium nitrate.

Hope this helps