I can't speak to investment banking, but Princeton student demographic data is available at
http://www.princeton.edu/pub/profile/admission/undergraduate... and the numbers actually surprised me a bit.
Looking at the hard data from 2012-13, we have 10.7% international students. So of the US students, 7.4/.897 = 8.2% identified as African American.
It's true that this is under-represented compared to the 12.2% of the US population for that demographic cited at http://en.wikipedia.org/wiki/Demographics_of_the_United_Stat... and in fact the ratio of the two percentages is about 0.67, so you could argue that about 33% of African American students who ought to be at Princeton if the student body were strictly proportional to population are missing.
If we look at the corresponding numbers of non-Hispanic whites, we see 72.4% of the population, and 56.7% (100 - 10.7 - 38.4)/.897 of Princeton's student body. The ratio is .78, which is definitely higher than 0.67, but not nearly as much higher as I was expecting when I saw the Princeton numbers. For one thing, I recalled the US as being 15-16% African American, but it looks like in the last 20-some years the demographics have shifted markedly... The upshot is that you see fewer black people in Princeton than you might expect based on general population but way more than you used to even in the recent past.
It would be interesting to see what US demographics for college-age folks look like, though; they may be significantly different from overall demographics.
For comparison with the above numbers, American Indian students are underrepresented at Princeton by a factor of 4 or so, and Pacific Islanders by a factor of 1.8. Hispanics are presumably underrepresented by a factor of 2 or so, but it's hard to tell with the complexities of the classifications there. Asian Americans are overrepresented by a factor of 4.
I'm not actually sure I have a point here, other than attempting to quantify "few"...