In the Carolinas, that blue line roughly parallels I-95. It was there long before any urbanization occurred, back to the 18th century, really. It’s mostly small towns still, though some have more recently morphed into suburbs with the northern influx. It’s the old plantation belt, mostly rural and agricultural, but quite black. Some counties are over 50%.
I expect interstates mostly follow the easiest route as did wagon trains, for example “the Whisky trail” that brought Scots Irish from ports in the North south to the end of the Appalachians and train tracks that went from the North around the end of the Appalachians and the west and northwest.