For what it's worth, my technique is as follows:
- Clean the item to be blackened. What I usually do is buff it with a steel wire brush, or scratch brush if it's small, to get the worst of any surface corrosion.
- Use an old paintbrush or cotton bud to liberally paint the blackening fluid on.
- Wait for as long as you think it needs to go black; for steel this takes only a few seconds, longer for brass and nickel silver.
- Set it aside to dry.
- Use a cloth or brass whizzydisc wheel to clean the surface residues off and buff to a shine if you like.
For Slater's wheels, a swift going over with some kind of abrasive brush is usually sufficient, followed by the fluid. Sometimes, it might need a second dose, but I always buff with the wire brush first. I rarely clean off the fluid, allowing to it dry before cleaning up things like treads of tyres.
It works for me. It's basically the technique outline by Martyn Welch. I sometimes wonder whether we overthink these things.