I took a look at my aforementioned Grand Prix, and I still suspect it's just oxidation. As I said, this spring wasn't totally black, but upon closer examination, the back of the upper arm - where I did not do any cleaning, sanding - is pretty close to what we can see on your spring. Other parts may have been exposed to more wear, probably that's why it wasn't as uniform. And this seems to suggest that the spring might not have been stainless in all cases in the past.
Hopefully someone with more experience with vintage models can say something more concrete.