I fetched another batch of surplus books from the University Library. "The Making of the Harlem Ghetto" by Gilbert Osofsky contains some interesting information; for example, New York's African community was founded by West Africans and not by native American Blacks. They were much sought after as household servants, replacing the less valued Irish and German. The gender imbalance was large: more females who had steady, relatively well paid jobs. There started the degradation of the Black male, who could not adapt to city life, and the disintegration of the family structure.
The massive immigration from the South was slow. Southern economy was based on African farmers - owners, shareholders, salaried laborers. Blacks grown up in slavery were appreciated, with their dignified humility and habits of steady work. All Southern States enacted laws against Blacks moving North and part of "racism" was, simply, resentment of their peasants leaving them behind. They tried to replace them with Italians, Portuguese, with total failure.
Harlem served for long time as New York's source of docile labor, but from 1930 the violence started with the introduction of drugs and political power (sale of votes). According to the book, till then Blacks worked very hard and earned their keep. One concludes that the current situation of the Afro-American population, where many live off the Government and enjoy special privileges, is something temporary and results from Blacks getting more organized than the general population (I think American Jews had a part in it) and exercising focused political power. But already Plato in "The Republic" said that total democracy cannot last and tends to evolve into an authoritarian regime. Maybe it is happening?
(Another question is what do I care? I am an old-old Jew in the Middle East, wasting time while lying to my desperate clients that I am working on their projects.)