The hood springs were not phosphate and oil. They were the medium to dark finish that spring steel naturally has when processed.
True. Spring get their "spring" thru a combination of materials, heat treatments and cooling.
There are a number of ways to replicate the finish manganese phosphate (dark) was mentioned. Gun blueing or Eastwoods quick black are others.
Gun blueing doesn't leave a reasonable reproduction of the heat treatment process. Eastwood metal blackening is about as close as you can get. Untreated spring steel, like what is used one hood hinges, has a tendency to "rust". There are ways to reduce that "rust" tendency, apply motor oil periodically, use Boeshield or WD40. (After I do a batch of phosphating, I apply WD40, as recommended by my mentor on the P&O process, a MCA Gold Card Judge, and by my own experience.)
Although I have never experienced a problem I have read that phosphating the springs can cause the metal to become more brittle.
There are at least two factors involved in spring deterioration, heat or a chemical reaction.
For the heat part, heating a spring to 500 F (or to red hot) and slowly cooling it will reduce the "spring" of a spring. That type of high heat does not happen to installed parts (except on EVs). Then left alone, the cooled part "flakes" and it loses the real dark color.
For the chemical part, phosphating has been used for decades and I've never heard of the P&O process doing that. (Note: I've been using the phosphate and oil process on car parts, mostly Mustangs, for over 30 years with no problem. I do periodically spray WD40 on my hood springs.)
Jim