Fixing the transformers would be the difficult part. Restarting a completely blacked out grid is a scenario we have plans for. Off the top of my head, I know

Fixing the transformers would be the difficult part. Restarting a completely blacked out grid is a scenario we have plans for. Off the top of my head, I know for sure that Bonneville Dam has black start turbines, and I wouldn't be at all surprised if all major hydroelectric dams provide the same ability.