Doug,
I'm glad that 625-650 works for you. I must be doing something different because at that temperature range I don' t get the correct blue-black color. I do get a very attractive dark blue at those temperatures. This dark blue looks very nice but doesn't exactly match the original blue- black color of my original vintage shotgun trigger guards. The color difference is less noticeable on guards polished at 400 grit or less. With my setup it takes at least 730F. This is just my experience. Thanks for sharing how you do it.