For the most part I would stick with what Peter stated, Though the 2006 MCA judging rule book states "semi-gloss black" or "phosphate and oil". The problem with these cars is that the finishes on some parts (shock tower caps, grill center supports, etc) will depend on where the car was manufactured and if the part was original or a service replacement part. The judging rules do not go into detail about what is correct for any given mfg plant, though a picky judge may deduct for the wrong finish.
If your bracket was originally black you should see some paint on the inside of the bracket. Phosphate will look gray and kind of sparkly.
but I know the gun shops bead blast and then dip them in a warm/very warm phosphoric acid bath before cleaning and then oiling the parts. Never seemed like a good idea to have toxic fumes in the house (maybe its just me). I do know different formulations of baths will impart different colors on different metals.