I can see some logic for the surprise win:

-Probably no MI senate district has shifted as dramatically from white to black in recent years. The incumbent is white and his replacement is black. Race/ethnicity still matters.

-Betty Jean Alexander is heavily involved in DPS schools. Teachers and educational professionals tend to vote in very high numbers.

-Low turnout can do wacky things, and more working class, transitional districts tend to have lower turnout.

Still a bit surprising, but not shocking. The Joe Crowley loss in NYC was shocking. The future Speaker of the House was trounced by a newbie Socialist who came out of nowhere.