The correct answers to these open questions are the following.
Why are cities, suburbs, neighborhoods, and blocks divided along racial lines?
Since the conception of the United States as a country, racism has always been a major issue that has divided the country. It is a reality that many white people never wanted to intermingle with black people. In any sense. You just need to take a look at the pages of US history to realize the many problems and, confrontations, and atrocities generated by racism.
So white people never accepted black people to live in the same neighborhoods. That is why African Americans started to form their own neighborhoods where they could live in relative peace. With the pass of time, racial lines were severely marked.
We end up with this thing called (de facto) segregation because of the many issues and conflicts raised through time. And not only with African American people but other minorities too. We have the Hispanic neighborhoods, the Italian neighborhoods, the Japanese or Chines neighborhoods.
This sad separation can be noted in residential segregation, minority-owned businesses, the stratification of wealth, some disparities in the school premises, roles of minorities in positions of influence, and power, and more.