I got this definition of culture from dictionary.com:

the customs, arts, social institutions, and achievements of a particular nation, people, or other social group.

Black culture is generally regarded as the culture of black people in America that are most likely descended from slaves. It doesn't refer to every black person in the world or every black person that is in America, it refers to a particular group of people that are also included under American culture. White culture encompasses a bunch of different cultures, like Irish and Australian and Dutch and German, while the term "black culture," when used in most contexts, means specifically the culture that descendants of slaves created, since there was no way of learning the culture of where they came from.

That's why I can accept that there can be a black culture and not a white culture.