Steve, The black oxide finish you are referring to has been called many different finish names. Phosphate and oil, or parkerizing/phosphating. I believe GM used black phosphate and have been able to duplicate this finish with a product "Chick" recommended, Palmetto dark,
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http://www.palmettoenterprises.net/Palmetto_Enterprises/-Welcome-.HTML."
Really pleased with the results. I had several nos GM bolts 30 years old that I had to compare to and it was a great match. I used an $18 crock pot with a ceramic bowl, worked great and very easy. If you search for Palmetto you will find more info. George