I will make the assumption that you are talking about entirely black wheels? I am under the impression that SAI in the 60's dipped and the magnesium wheels in a
zinc type solution to help avoid oxidation. Then they would polish the edge. This is the reason the centers turned gold and the edges silver. Once oxidation occured the edges looked more like they were primer grey.
That being said brake dust and magnesium turns black. Example CSX3026 in the Youtube video with FIA mag wheels. Remember these were race cars so cleaning and polishing wheels were low on the list of "to do's". Many of the wheels were painted white, FIA, and the rest black or grey centers if not left as per OEM.
CSX3020 with Magnesium Halibrand:
http://www.clubcobra.com/photopost/s...0&ppuser=22813
CSX3102/3005 black
I think aluminum Replica wheel)
http://www.clubcobra.com/photopost/s...0&ppuser=22813
CSX3197 with I think Magnesium painted wheels:
http://www.clubcobra.com/photopost/s...o=40901&cat=15
I think CSX4027, Steve Sunshine, has the centers all black. Great look and painting can bring the vintage look to life without years of setting.
Maybe someone else can clarify.
Jeff