Keith I agree he, Doug, was very talented but he took a lot of extra effort that we just do not appreciate. We got to see his final efforts when he had it all figured out. I'll never know how he got the results he did but I do see a few British blackeners who do work of the same quality high gloss deep black finishes.
It must take decades to get it figured out and I would not expect anyone who does that to share more than very general suggestions. After all, it is their livelihood, not a hobby, and once out there everyone could duplicate their work for free. Doug could have sold his formula but could he sell his attention to detail and his technique?