My guess is that those radio stations are successful because of listeners who live in the suburbs. For example, Detroit's population is overwhelmingly African-American (82.7%); however, the Detroit metropolitan area is 70.1% white.
Radio does not think about city limits. Radio is programmed and sold to advertisers based on a defined Metro Survey Area.
The Detroit MSA consists of Lapeer, Livingston, Macomb, Monroe, Oakland, St Clair, Wayne counties.
That metro is 22% African American and just under 5% each for Asian and Hispanic.