Why did the term “Black Belt of the South” have a double meaning?

"I have often been asked to define the term "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." Chapter 7

@Aslan - I don't know if I'm suppose to do this and if so I'm sorry but all I wanted to say is thanks for the information.

No worries Randall, anytime.