Obviously it depends somewhat on what the plan actually is, as to how complete the blackout is.
However, in a major terrorist attack or an attack by another nation on the continental United States, shutting down the majority of the internet would be precisely the worst thing to in terms of economic impact.
This seems self defeatist when the justification I found after a brief
google was "The Senate has tried to downplay these concerns by saying that “only specific systems or assets whose disruption would cause a national or regional catastrophe would be subject to the bill's mandatory security requirements.”
So... because you're scared of cyberwarfare you're going to turn the internet off?
Surely it'd be better to use these over-arching powers and fuck with the aggressors communications networks (which is easy against a sovereign state, you could fuck up there TLD resolution, and perhaps use QoS or firewalls to blast any traffic headed for or out of said country traversing your network - all without so much as a cruise missile), but somewhat harder against terrists.
It should be worth mentioning that most recently China looked at getting it's own Domain Name system, effectively creating a new internet - it's an idea that's been floated in the past but never (commercially) successfully implemented.
The other option would be to physically turn off or unplug the data centres at the ends of the various cables joining the United States to Europe and Asia/Pacific, along with any public/commercial satellite relays, and turning off any root domain controllers which reside in the US (I think all the high ones do). I can't see what you'd achieve though, despite the US Government having alternate means of communication.