So in theory, a "natural" number would be to have around 13 black students for every 100 at every university. If it is less, then it may be due to either a) Lack of opportunities or b) Lack of desire enroll in specific subjects.

Here in Mexico face a similar issue with the lack of Women in technical careers (programming in my experience). The fact is that for some reason less women enroll in Soft. Eng. or Comp. Sci. courses... And from the few that enroll, several get out after the first or second "filter subjects" (Data Structures, Programming I, algorithms). Given that there are more females (61 million) than males (58 million), in theory there should be roughly the same number of both in schools.