Therein lies the rub. There are no Blue States. There a Blue Urban areas that have enough people, maybe, to tilt the politics their way. The majority of the respective states are conservative. Aside from maybe one or two in New England, damn every state is predominantly RED.
Let’s say, God willing, this happens and Illinois secedes. Take away the three big urban areas out and Illinois is about as RED as it gets. Virginia, NY, California, Washington, Oregon, Colorado, etc etc etc. what happens when all of Pennsylvania, except for Philly, Harrisburg and Pittsburg don’t want in on the Blue State Secession. They want to stay with the folks in the RED areas.
Is there any reason that a state has to have contiguous territory? Could New York City, Los Angeles, Chicago and San Francisco be consolidated into one political unit, with all of their congressional seats and two senate seats? Yeah, I know, never happen but I can dream.
If a majority of a state votes for secession then it goes. Secession is color blind. It doesn’t care whether you live on a farm or in an urban project.