[M0TGX Terry]

I also have a 'brass blackening ' job awaiting resolution. It's the bezel around the dial aperture on an Osram Music Magnet. A horologist aquaintance kindly sent me a list of various chemical solutions for colouring brass and other metals, copied from a book in his possession. The book dates from well before the second world war. Sadly, the recipe for blackening brass begins with half a pound of white arsenic, which appears not to be available at any of my local chemists!
A horological supplier may be a source of something suitable? I've not pursued it yet as this particular job has slipped down my to-do list.
If anyone wants a copy of the relevant formulae (if the forum rules permit) I can post it tomorrow.