Under the circumstances of a real cyber attack, the precious items you rely upon currency of any kind to buy will be unavailable.

The point is moot. Those who do not have emergency supplies for a complete blackout are not going to be able to find those supplies when the blackout occurs, no matter how much paper money they have.

When the lights go out, all currency will be equally worthless. However, when the lights come back on, I contend that it will be the crypto currencies that will have appreciated in the interim period, while fiat currencies will have inevitably depreciated.