Detroit then and now: An infographic.

[hudkina]

It really started in the 1920's, but picked up steam during WWII when a huge number of working-age men were fighting overseas. Blacks were able to obtain the types of jobs normally reserved for whites, especially in manufacturing cities like Detroit. After the war ended, and many of the white men came back expecting their jobs back, there was a bit of push-back from a growing and emboldened African American middle-class. That push-back, eventually lead to the Civil Rights movement of the 50's and 60's.

Because Detroit was one of the jobs bright spots of the War era, it received one of the largest shares of black residents. This allowed a black culture to flourish in the city that by the 60's and 70's was drawing blacks from all across the U.S. This culture bred among other things the Motown sound. Think of it was being the "Atlanta" of that era.