I hesitate to answer this, because it's going to require a lot of generalities and obviously shaped by my own personal experiences. So read the following with that in mind, I guess.

Blacks in the USA definitely do have their own culture. How much it differs from mainstream culture will depend on a lot of things - income level, where you live, your own family makeup, etc.

While they did not bring much (or lost) their original cultures, the separation of blacks and whites during much of American history created a distinct culture within the black community. Historically, that separation was forced (slavery, jim crow laws, racism in general) but in more modern era what remains is mostly voluntary (media, music, organizations).

I grew up in an area that was pretty segregated, whites in one area and blacks in another (in a northern state) but by high school we all attended school together. My black friends definitely had different fashion, slang, music, and family experiences than I did. (attending an all black gospel baptist church as a white girl is quite the experience, hehe)

I had black friends who were accused by their peers of acting white and of course white friends who were accused of acting black. Even as kids we recognized the differences in our cultures and the stereotypes we had attached to them.

Perhaps sub-culture is a more accurate label, I'm not sure. But on the whole, there are definitely differences in mainstream black and white culture in the USA.