I've heard of the "bolt trick", folks say it was listed in one of the "Gunsmith Kinks" books. I don't have the books yet, but there seems to be lot's of info in them.
Ken, that's the understatement of the year. Four volumes are packed with tips and techniques from guys who make a living working on guns. Of course, some of it will be all but useless to a guy who works on nothing but old doubles, but when you get the complete set, you will kick yourself for not doing it sooner.
I suppose you could overheat a nitre bath, but just dropping the heat after it completely melts will keep you in the proper range for turning steel blue. The infrared thermometers seem to be quite accurate if properly calibrated, and you could certainly use it if you have one. But I wouldn't go out and buy one specifically for that sole purpose. I've heard that the nitre bath is also very good for tempering springs after hardening, but I haven't tried it yet. Have you?
