I imagine in most black start scenarios the coal plant would already be warm. If black start takes a day and the coal plant cannot operate without external power then maybe not so much. I don't think you need spare power plants so much as backup generators that can provide internal power to start a black start capable power plant up. If you do not have enough black start capable power plants that is not a good thing of course.
But it's not just that: to black start the grid you have to shed load - that is, you need to trip all the breakers along the grid, and then very carefully restore loads to avoid knocking yourself offline again.
You just plain aren't going to do that in an hour when you're offline because someone trashed your automated systems - resetting and restoring the grid would require people on site, in communication. That's not happening in an hour without staggering expense.
