I believe axle housings were painted [glossy black] immediately after machining. Assembly paint daubs were later applied on the axle paint. Since the axle paint wasn't very good it did not adhere for long particularly in rust belt states. Brake components and axle parts [flange, etc] should not be painted. The rear axle cover and backing plates were likely received already painted. In my opinion cast-blast is not correct for axle housings.
Another common error is cast-blast for rear brake drums. There are photos in several vintage magazines showing the installation of t-bars or gears; drums were semi-gloss black.