You can't do a black start from the lowest potential part of the grid anyways. Synchronization isn't the hardest part. You could solve this with GPS and cellular telecommunications. Fault current handling is where things fall apart. You have to start the grid from the top. Fault current has to flow in the right direction or none of the existing equipment will function correctly.

The power walls can help to stabilize islands once they're back online, but they can't meaningfully participate in the initial phase.