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There is a chemical sold in the UK under the name of Tourmaline that it used to blacken brass and copper. The item to be stained will need to be completely grease free for the chemical process to work.
Good old nitric acid will also work.
Teaching my granny to suck eggs, I'm sure, but please ensure that you wear latex or nitrile gloves (some people allergic reaction to powdered latex) and eye protection if you go down either route...
You can use a chemical called black max. It can be ordered from a jewelry making supply website. Make sure to tape off the surrounding areas to avoid getting the liquid on them.
He mentioned that it's just the puller, so presumably it could be removed from the jacket and dipped in solution if need be. Should make the process a bit easier and not so messy.
Thanks guys! Any more ideas are welcomed. I just have the brass Conmar bell puller to do then I will install it, so no danger of splatter (except onto myself of course)
Also you need to make sure it is completely free from grease or dirt, like fingerprints. This will block the blackening solution from making a good reaction if it isn't clean.