There were few Blacks in Columbia and Harvard at that time so this is not exactly like remembering a couple of random White guys like you or I.
I remember the Black guys in my graduate programs because they could be counted on one hand. I sure this would be the case with WhatsHisName? just about every college he has been.
The African kid in one of my Econ. grad courses made an indelible mark on me. He looked absolutely terrified and was sweating bullets for some reason.
Foreigners in American colleges stood out in those days. African-Americans, even in the early 80s, probably didn't -- not so much -- especially if they didn't say much.
FWIW, if Obama did apply as a foreign student, he probably didn't keep up the charade. You can imagine how tiring it would be to always have to talk in an accent.
And of course, college is different from graduate school, where classes are smaller and people in the same department are more likely to take all the same classes.