Complete blackstarts are scary stuff. We have all this infrastructure to assist with pulling one off, but it's not the kind of thing you can test. As they saying goes "your backups are only as good as the last time you tried to restore them."
For example, you can't just turn on all the power plants. Ignoring the phase issue, power plants require some power to operate (which is where the flywheels and batteries come into play). This means you need to isolate them, and bring them up one-at-a-time (and you probably want to isolate consumers until the whole process is complete). How do you coordinate this in the absence of power/internet? Radios, but now you have to worry about the logistics surrounding that. Do you have a stockpile of petroleum that can be tapped without power, because your people aren't getting on-site in a reasonable time without a car. How familiar are people with the process, having never practiced it?
Couldn't you test it by either isolating an existing region and try to blackstart that particular region, intentionally out of phase with the main grid, and then try to synchronize and connect.
Black-start-region and join-regions should be enough to get a whole grid going?
If you can handle the consumer load in the region without local plants, maybe? That would still only be a 90% test because you can't completely blackout a region for testing purposes, consumers would get angry.
For example, you can't just turn on all the power plants. Ignoring the phase issue, power plants require some power to operate (which is where the flywheels and batteries come into play). This means you need to isolate them, and bring them up one-at-a-time (and you probably want to isolate consumers until the whole process is complete). How do you coordinate this in the absence of power/internet? Radios, but now you have to worry about the logistics surrounding that. Do you have a stockpile of petroleum that can be tapped without power, because your people aren't getting on-site in a reasonable time without a car. How familiar are people with the process, having never practiced it?