Most folks would accomplish this same thing using Nitre salts. Going above 700-800+° for a few minutes will give you a nice black, uniform finish. I’d rather use a medium like nitre or molten lead to achieve this. I think total, uniform immersion into the medium is a more conducive to finer, more repeatable results.
I would only ever use a touch on items no larger than a safety button, etc. Parts much bigger than that, using a torch is hard to evenly heat the piece to color it and get good results.
Your method is interesting though, no doubt about that.
