Lots of African Americans have some European ancestry, as well as African ancestry (some also have Native American ancestry). So it is not news that lots of African American people (students or otherwise, at Harvard or otherwise) have non-African as well as African ancestry. It is only because of American social convention that such people are typically seen as exclusively African American rather than multiracial.
http://www.cell.com/cms/attachment/2028657465/2046793154/mmc2.pdf describes the often hidden or ignored multiracial ancestry of many Americans.