A few years ago, I met a former Hammtramck autoworker and was asking him about this; he laughed and said there was no procedure to paint the springs, they were installed bare metal...but during any type of equipment down time or slow period, their managers would have them clean up, organize and spray paint any of the parts sitting on the shelves...getting surface rust, etc... This would apply to more than just the springs of course, but he said that if the leaf springs were just sitting on the shelf acquiring surface rust after a week or two...they would get spray painted black when the work slowed down...
That would account for body-colored paint runs over black paint, etc...