Bluechip -- Parkerizing is a heavy phosphate coating (DOD-P-13232). Black oxide finishing is an oxide conversion coating (MIL-C-13924). They are different finishes.
Anthony -- It depends on what you want the black finish to accomplish for you. At the simplest level, heating the part up to the very beginning of a "glow" and dumping it in dirty oil and then reheating it until the oil "flashes off" will give you a finish that will last for many years if no undue rubbing is involved. I have tools that I made in the mid-1960's that were treated this way that have no rust whatsoever on them. This finish will not stand up to friction or heavy solvents (alcohol is fine, acetone is death). (US) Military armorers (read: gunsmiths) thoroughly clean their parts with acetone, dry them in an oven at 250°F, and then place them in a sealed container with a couple of drops of fuming red nitric acid for a few hours as the pre-treatment. The part is removed from the sealed container, exposed to steam, and allowed to sit in a still air location until and even sheen of rust has formed. The part is then dunked in baking soda for a day or so (to halt the acid). It is then boiled in a pressure cooker which will turn the rust to a black iron oxide coating. It is polished (and, almost always, waxed) before being presented to the customer. On the other hand, I have had good results with the Birchwood Casey & Caswell cold black finishes for steel (be sure that the lye has been well sealed against moisture). The "Precision Brand" process has been a complete catastrophe both times I have tried it.
Just about any gunsmith worth his salt will have tanks set up for both Parkerizing and Black oxide finishing -- you may have to wait until he has enough parts to process to get it done at less than a rather high price.