They're generally viewed as being the same grade of stone, just different colors. I've read a post where Newt Livesay said the black stones are better, and once A.G. Russell wrote that he'd found that the black stones, by weight/volume to be a slightly finer grade. I think when you get to that level you won't notice much difference. There is also a bit of overlap with the stone qualities in regards to color. I have a gray, non-translucent arkansas stone that's at least as fine as my black one.
BTW where did you get your Norton black arkansas? I can't find any on the net.