I have a little experience with nitre bluing. I have blued screws, pins, and even a couple of grip safeties. I used a small metal dish on top of an electric range. It was just barely able to reach the necessary temperatures, and I did it in the dead of the winter, so I wouldn't over heat the house. I think my results were pretty good.
I have heard of others using your plan. One fellow used a Lee casting pot filled with salt peter, and said he got good results.
If I understand your post, you are thinking of using lead. It should be easy to obtain a consistent temperature with this approach. I meant to try it last fall when I was casting bullets, but other things got in the way.
Be sure and let us know how it works out. Steel floats in lead, so even if it doesn't work, you have only lost a little time from a bullet casting session.
Different alloys melt at different temperatures, some thing to consider.