Then is it fair to say that you do not identify as white or Caucasian as those are also recent social constructs? If this is true, then Blacks (generally speaking) were not barred from holding priesthood because of the color of their skin. There had to be an obvious reason for the ban since black people weren't marginalized for being black.
Here is a great article to be read: The Social Construction of Whiteness: Racism by Intent, Racism by Consequence