There would be ripples of course, and it's not to say there won't be significant impact, but I doubt we would "go black" even if we lost the whole east or west coast. The systems simply aren't constructed that way. Having worked within a utiltiy kernel, I know that systems can switch out to redundant and independent backup, depending on how well the utility services have invested in the back up plans. In some cases, you could lose all outside support and they would still be indefinitely operational on their own. There are enough of these set-ups that it would take something much worse than a 15 story wall of water washing out the coast to overwhelm our infrastructure.

This is not to say that such an event is insignificant or that it won't cause widespread problems, but I would really only be concerned if I were in the immediate area of effect. Otherwise it will really only be an inconvenience for most of us. If I am in the flood zone, then the obvious response is to leave before it floods.