unfortunately, ethnicity does exist. i don't think it's easy to get rid of the division for the very cultural reasons that create it in the first place. people identify with their ethnicity often, for better or worse. it's hard to shake or just wipe the slate clean when culture or social group ends up tying into a person's sense of their own identity. that there are patterns in culture/ethnicity makes it even more difficult because people will misattribute them as fixed properties.
also the documentary i watched, everyone in that neighborhood was black for the most part. black doesn't simply mean a certain set of patterns in physical features. the people identify themselves as this. but what's more important is the ethnicity and the history of passing culture over the course of generations... which is rooted in the conception of race all the way back to slavery. thus you kind of have to consider it when considering the problem.
like if my great great great grandmother was denied access to knowledge, born a slave, and lived in poverty after the civil war ended (just expected to magically know how to integrate with society); and maybe her kids learned a little bit more than she did, but still were largely ignorant and living in poverty... imagining what is passed from generation to generation, landing with me living in the projects... it's these subtle things in the worldview that end up impacting people more... now you have more access to knowledge, you are literate... but you are still trapped by an disempowering worldview that gets passed on and elaborated upon per generation. one based on being a slave in origin.
i guess as an example, alcoholism gets passed generationally in both sides of my family. it's not simply people drinking, it's a sort of cluster of behavioral traits that can appear in children of alcoholics that they then unwittingly pass to their own children without trying to, even if they themselves are not alcoholics. the pattern of traits can then make it easier for someone down the line to develop alcoholism. (there's probably some genetic thing too though maybe in this case.)