My 69 A body is phosphated or Parkerized. I'll bet this B body is also. Parkerizing is the dark gray - black finish on military weapons since before WWI.

You can Google Parkerizing for more info and find sellers of the chemicals.

Almost all fasteners on your muscle car that aren't zinc plated are phosphated. Also 4-speed linkage and front drum backing plates, as well as many front end parts are too.