Cities need everything
Virtually all cities get their drinking water from somewhere else, as well as their fuel, electricity, natural gas, food, and damned near everything else needed to support life as we know it. Red county farmers can still sell to the millions of people living in other red counties or export their products overseas.
To be sure, people living in the red counties would feel economic impacts if they lost the blue cities. They'd also lose the majority of their entertainment. Without the red counties, the blue cities would starve to death in the dark, unless they're willing to pay through the nose to import everything needed for life from overseas.