If the “Blue” States Secede …

… what would keep parts of said states from seceding from them?

One of the reasons for the proposed secession is that the blue states send more to the red states than they receive. I supect that it's only a few parts of the blue states that are responsible for that and they might secede in turn.

What would keep Wall Street from seceding from New York or Beverley Hills (where the Republican vote doubled in the past four years) from seceding from California?

There's a better solution to the money leak out of blue states. We can vote for politicians who will push through tax cuts for the rich. As Lacey Davenport's husband once said (as reported by Gary Trudeau): “Try not to forget we're rich.”