Northup refers to himself and other blacks in what we today would consider demeaning terms. What do you make of these references to blacks?

These sort of references continue today among blacks. The slur takes on a slightly different context when used within the culture that is being offended. Some blacs feel a sense of empowerment using these slurs on each other. There is much to this question so I'm going to provide you with a link:

http://africanamericanenglish.com/2010/10/22/why-black-people-can-use-the-n-word-a-perspective/