From The New York Times:
The decision was handed down by an unusual coalition of justices, and was the latest in a series of setbacks for Republican-led legislatures. In recent cases concerning legislative maps in Alabama and Virginia, as well, the Supreme Court has insisted that packing black voters into a few districts — which dilutes their voting power — violates the Constitution…
Justice [Elena] Kagan said black voters, in coalitions with others, had been able to elect their preferred candidates even before the redistricting. Adding more black voters to the district, she wrote, amounted to an unconstitutional racial gerrymander.
North Carolina and Texas are the only states where courts have ruled that the new voting restrictions and redistricting maps that came afte 2010 were discriminatory against African Americans, reports The Nation.