Tried it a while ago when making packaging machinery components, cor blimey expensive liquid, that stained everything it touches and didn't last very long on the components, local heat treaters did a far better job, I wouldn't use it again.
Blacking, bluing or japanning?
The 'FIXER' chemical used in developing black & white photographs can be used as a blacker for steel items. Get them very clean, heat the fixer to just under boiling and suspend the part in it. Dunk time determins the depth of black. Use pyrex vessel not a metal vessel to heat the fixer. Take out, dry and lightly oil.
Presumably you can still buy fixer from chemists and the like - it used to be very cheap.
The method I have used for the last 50 years is to hang the item from a hand held wire,Have a metal container with old engine oil deep enough to immerse the item. Then heat the item with a torch till its hot enough to produce a good smoke when dipped in the oil (but not hot enough to cause flames) Just keep heating and dipping until you have a lovely smooth satin black finish.
Want to re-black some old tools from a motorcycle toolkit. I've seen some diy cold chemical treatments for this purpose, has anyone ever tried these or have any experience of the process ?
I'd have thought that would be more for quench-hardening?