Ken, I've used one many times and own one, but never thought of using it as you suggest. By nitre blueing, I assume you are talking about immersing polished steel parts in a molten bath of saltpeter or potassium nitrate. If so, just bringing the pot to a temperature where the potassium nitrate is completely molten will have you at a temperature that will turn the parts blue without the need to monitor the temperature. Once molten, just back off on the heat enough that it doesn't begin to solidify.
Have you heard about the little trick of inserting a long lag bolt into your nitre bath before it cools and solidifies, and then unscrewing it to leave a vent hole to prevent pressure build-up the next time you heat it. A rupture through the still solid crust at 633 degrees F will ruin your day. Just another tip from Brownell's "Gunsmith Kinks" that makes those books worth every penny.
