In the late 1930’s, as Detroit grew outward, white families began to settle near a black enclave adjacent to Eight Mile Road. By 1940, the blacks were surrounded, but neither they nor the whites could get FHA insurance because of the proximity of an inharmonious racial group. So, in 1941, an enterprising white developer built a concrete wall between the white and black areas. The FHA appraisers then took another look and approved the mortgages on the white properties.
If you haven’t already, def. check out David Freund’s Colored Property. Beyond its discussion of racial covenants, it also offers a pretty great critique of Jackson’s Crabgrass Frontier (quoted above) and challenges some of his assumptions re: the inexorability of so called “white flight” and the particular ways in which US metropolitan areas were racially reconfigured following the Depression.