Although your precise wording is correct ("prominent understanding"), far greater splits exist, for example, on political views and on income, and arguably even more harmful.
I focus on race because it's an entirely artificial construction. It makes sense that, say, communists and capitalists might want to live in different communities (and it's not clear it's a bad thing). There are also forces that combine to push people of different incomes into different areas that aren't related to a desire for cultural homogeneity.
At least where I live, the segregation isn't on skin color, although it sure looks that way at first glance.
There are two major communities (African Americans and whites) and a number of smaller ones.
The "white" community actually refers to "WASPs and their descendants," but a number of other people get lumped in based on skin color. The AA community refers to descendants of slaves.
The two communities speak different dialects of English. They have drastically different communication patterns (when do you use last name versus first name? when is it okay to yell? how do you show respect?). They have different religious practices. They have different practices for raising kids. They work different jobs. They differ, significantly, on almost all cultural dimensions.
I see very little discrimination or desire to segregate based on skin color. If an Ghanian moves into the community, they're more likely to move into the white community than the African American one. Likewise with African Americans who grew up in WASP communities.
In practice, many immigrants -- of all skin colors including whites coming from places other than Western Europe -- tend to form their own community; they don't quite fit in in either the "white" and the "AA" community. Both communities accept a narrow range of behaviors as appropriate; the broader international community tends to give a bit more cultural leeway. A Nigerian, a Japanese, a Russian, and an Indian immigrant will interact with each other just fine, but quite often don't assimilate too well into either "white" or "AA" communities.
Distinct cultural communities that often, but don't always, correspond to ethnicity is exactly what you would expect from the construction of a "racial" hierarchy.
If we divided caucasian Americans into two groups, designated one group as "lesser," and then waited a few hundred years you would find similarly stark cultural differences. Again, race is entirely artificial. People were divided based only on superficial appearance and, once those divisions are enforced, cultural distance grows.
So, what you're experiencing is the outgrowth of a racial classification system. It's entirely artificial but, as you say, it also has real consequences. Even if, by magic, we were all transformed to have the same ethnic heritage, the cultural legacy of "race" would make "race" very apparent. African American and caucasian American culture is different - even though those differences are partially (largely? hard to say) from the racial structure imposed on African Americans.
The answer is in your final paragraph. Neither "white" or "AA" culture is a perfect fit for new arrivals (just ask Jamaicans or darker skinned Dominican ex-pats how well they feel seen by any American culture). Indeed, no single cultural approach is "right" and we do ourselves a disservice by searching for one. We have had structures and policies that have driven the two largest cultural groups (white and African American) apart for generations. Abating that push should be a high priority. We also need to be attentive to including as many cultural groups as possible in finding common ground and understanding for living together comfortably. All groups will need to "give" a little ground in terms of what they would most prefer and, in the interest of equity and facing history, many experts think caucasian Americans should give the most.
