This isn't shocking. Black voters have voted en masse for the Democratic Party since the mid-60s and the passage of the 1964 Civil Rights Act, the Voting Rights Act and the social welfare programs of the Great Society. This solidified black voters behind the Democratic Party, but they had been moving there since the New Deal.

However, it's a historical anomaly in the United States. The traditional home of the black voter was the Republican Party, due to its historical role in ending slavery and introducing Reconstruction Acts and Amendments to the Constitution. It also did not help that the Democratic Party was the party of Jim Crow, a system of legally enforced segregation present throughout the American South in the aftermath of the Civil War.