They'll have to sell whatever they grow on their farmlands. Who do you think they'll sell to? To other farmers from other Red states who are also looking to sell what they grow too? You also forget that both CA and IL (Blue states) have farmlands and are among top food producing US states.
And it's very likely the farmers in CA and IL are conservatives. What's stopping them from fleeing? Who runs their farms then? Or what if they refuse to sell? Or worse yet, what it they torch their farms as an act of defiance/sabotage before they go?
How do CA and IL feed the entirety of the blue states? How do they get this food to NY, for example, if the red states block commerce or simply seize the goods? So you're looking at armed conflict at a minimum. And this is just the tip of the iceberg in terms of the logistics involved to simply FEED people. What about water? Energy? Medicine?
Both red and blue states would turn into 3rd world hellholes overnight.