
Which area in Washington DC had developed as a predominantly black neighborhood
As Washington became progressively more segregated, the U Street Corridor and neighboring Strivers' Section emerged as fashionable neighborhoods for Washington's African American residents.
It became the city's most important concentration of businesses and entertainment facilities owned and operated by blacks, while the surrounding neighborhood became home to many of the city's most prominent African Americans.
http://www.culturaltourismdc.org/information2550/information.htm?area=2529

