With nitre bluing, you may be looking at heat color of steel. It can be used to produce other non blue colors based on temp changes. For both getting the right color or using it to nail the right temper for a spring, I'd consider monitoring the temp more accurately.

I'd think a good conventional thermometer or thermocouple in the correct range would get you closer to where you want to be. If you weren't aware of it, Brownells has a good pdf of the nitre bluing process even if you decide to go the stump killer route. Only thoughts, and best of luck with it.