The Black Belt is a region of the Southern US with a history of slave plantation agriculture and a high African-American population.
The term originally referred to a specific stretch in central Alabama known for its dark, fertile soil. That stretch eventually became associated with the slaves who tended to the land, and the term expanded to include the greater region where slavery and cotton farming was widespread.
Here's how Booker T. Washington described the evolution of the Black Belt:
So far as I can learn, the term was first used to designate a part of the country which was distinguished by the colour of the soil. The part of the country possessing this thick, dark, and naturally rich soil was, of course, the part of the South where the slaves were most profitable, and consequently they were taken there in the largest numbers. Later, and especially since the war, the term seems to be used wholly in a political sense — that is, to designate the counties where the black people outnumber the white.
Today, the term Black Belt generally encompasses a stretch of counties from Virginia down through the Deep South and including parts of Texas and Arkansas.