South Carolina used to have a black majority during Slavery and Reconstruction but it ended around 1950. The black population is still large but not quite enough to override the white majority politically (same shyt in Mississippi).
The reason is simple all over the South: 90 percent of blacks vote democratic but the same percentage of whites vote republican. Race is the best indicator of one's political leanings in the South today.
For example, Obama got 95 percent of the black vote in Mississippi, but it didn't matter because 91 percent of whites voted for Romney.