Some have, some haven't, just like with most groups that are or have been outside of the mainstream. Clearly many African Americans have assimilated, moving into the middle class, living in the suburbs, etc. However, there are some who have not. These are typically people who, for whatever reason, live in very black communities where there is not a lot of influence from "mainstream" American culture.
As a whole, though, African Americans have assimilated fairly completely. However, it's also worth asking how much of that has come as whites have adopted aspects of African American culture.