In order for either country to function there will have to be considerable trade. I think the point is that support for Redmerica will then be targeted; Bluemerica will only pay for what it needs.

However, I thing the the truth is that Bluemerica will keep propping up the Red because liberals will feel bad for people trapped in Southern cities and send foreign aid. Squeezing POCs will become a money maker, so by the rules of capitalism they'll do it intentionally, human cost be damned.