The Black Belt of the United States is comprised of the deep Southern states, primarily the Carolinas, Georgia, Florida, Texas, Alabama, and Louisiana. They were named for two reasons: first the soil of the area was traditionally quite rich, lending itself to the agricultural economy which dominated the area for many years. The blackness of the soil--an indication of its fertility--led to the term "black belt."
More consequential, and related, is the fact that the same area contained the largest population of African Americans. The agricultural basis of the economy of the region meant that large scale plantation agriculture, and consequently slavery thrived in the area; hence the large number of Black Americans, most of whom are/were descendants of slaves. With the possible exception of the Exodusters of the Reconstruction era, a substantial majority of the Black Population of the U.S. prior to World War One was located in the deep south. The meaning of Black Belt is often used with some degree of double entendre, either of which is appropriate.