An alternate is to nitre blue using molten potassium nitrate salts..
I do this for small stuff like screws, sights and triggers..
Use a strong seamless or welded can and melt the salts in it. Be careful to put a lid on the can when reheating a solid batch as the salt will foam up and "fountian" out the pot as it breaks through the crust.
Tie the item to be blued with a bit of soft iron wire and dunk it in the salt. Watch it go through the colour changes and hoik it out when you get the colour you want. Dunk in water.. Done!