From what Du Bois says in Chapter 7, we can see that the "Black Belt" is a region in the deepest part of the South. Du Bois talks about it specifically when he is talking about Georgia. He mentions that it starts "below Macon" and that Albany is at its heart.
The term "Black Belt" is said to have come not from the color of the skins of the inhabitants but rather from the color of the soil. It is said to be an area of very dark and rich soil. However, the term also does have a racial connotation. This was the area in which cotton was "king" before the Civil War. This meant also that the area was heavily African American after emancipation.
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.
The “black belt” was in the region of the Deep South with the highest concentration of slaves. It emerged in the 19th century as cotton production became more profitable and slavery expanded south and west. The lot of the slaves was harder here than in the more settled areas of the Old South. Stretched from SC & Georgia into the new southwest states of Alabama, Mississippi, Louisiana.