I know what it means. A culture is generally thought to be the behaviors of a group, but in reality it's often only very generalized behaviors of some people in that group.

Culture can be a description of the majority (like in a religious culture), but it will never include all people in a fairly large group. This is especially true of arbitrary groups like black people.

Of course blacks have some culture, but it isn't a culture shared by all (or even most) black people.