An addendum to my post above...as I thought of it...ISPs (under coercion from .gov) could also blacklist a certain number of sites (Twitter/Facebook/Livestream/GMail/etc.) locally pretty easily, without totally blacking out the entire connection to the web. Turning OFF upload bandwidth locally (i.e. making the ISPs internet connections read only or download only) is also possible.
While all that is possible it is also trivial to work around. No download can take place without an outgoing TCP connection and "upload" of a request. Therefore all desired uploads can simply be inserted or steg'd into requests depending on the efficiency tradeoff. As for blocking sites, talk to the Chinese, then talk to the people who work around the crap that the Chinese do. It is a fair amount of work and an arms race, but the information access side always wins unless the internet is completely turned off.