The black dots represent cotton production in the 1860s—each dot is 2,000 bushels bales. The blue counties went for Obama in 2008.
One of the commenters on the excellent Strange Maps site, whence this map came, notes that the history of this slice of land goes back even further:
These areas are still used predominantly for agriculture, and they actually have a name: the "black belt," which refers both to the region's rich, loamy soils and to its demographics.
So, in addition to seeing this swoosh-shaped pattern in political maps and in maps of 1860 cotton production, you'll also note it in soil maps and in geological maps of shorelines in the Cretaceous Period.