As far as to answer your question of specific processes to acquire the "correct" finish for the springs, with the exception of paint color indicator(s) for application identification the springs I believe since you were asking of how to recreate the production finish, perhaps they were "natural", unpainted otherwise; yes?, no?, I think so, but not betting my meager existence on it!; somebody help out here, before I bury myself!
If so, the problem is what is the "natural" finish appearance? In most instances spring steels post heat treat are shot-peened to clean the surface of the impurities (e.g. scale, etc.), blend imperfections in manufacturing (e.g. scores on the surface from the drawing in the forming process, general deburing), and for the further treatment of the surface as in compacting such to add additional resistance to fracture formation, and etc. (yep, I didn't include everything, but........). In this process most if not all of the coloration from the heat treating is lost (unlike the color case hardening on ones' 1886 Winchester lever action), and one would be left with the effect of the heat generated in the deformation process and in trading of surface material mixing with the base material, creating a color of steel of neither.
And looking at N.O.S. or original unrestored components, unless sealed against oxygen contact since produced, in paint, & removed in such a fashion as not to change the coloration, & some surface coatings do breathe, grease/oils, although in time this may stain the surface causing discoloration, as petroleum products do oxidize, and before one says nay, what do you have to compare it to?
So? I don't think I helped, but I was trying to perhaps provide a better understanding of what exactly would be the right finish?
O.K., so after all that B.S., were they just painted some sorta black?
Scott.