Who is Negroes from A Walker in the City and what is their importance?

Throughout the book, Negroes play a peripheral, but undeniably present, role in the action. They are frequently referred to as passing by the author's home on their way to their own neighborhood, and if the author is to be believed, always did so singing. Later, they are also referred to as making what the author suggests are resented incursions into the Jewish neighborhood. Initially, these "incursions" come from Socialist activists with a particular agenda (to raise awareness of the plight of Negroes in the American South), but then, in the context of the 1930's Depression, evolve into a stealthy invasion of what comes across almost as scab labor, accepting lower wages than unionized workers like the author's father.