Blackening Brass - Any Recipes?

I'm in the process of making a couple of threaded brass rings to facilitate the installation of the LED illuminator Stuart very kindly made for my centring microscope. The rings, roughly 32mm OD, 8mm long with a wall thickness of around 1.5mm and threaded, need to be matt black.



I thought Liver of Sulphur would do the trick but my daughter reckons the stuff doesn't take to brass too well though it's great on copper and silver.



Any suggestions?



Bob.