Considering that the point of the blackout is to raise attention, hiding is already enough. Anyone who gets to the content will be aware of the SOPA this way. I guess anyone can just use non-US proxy to get to the content anyway.
I'm sure there will be many non-US people wondering why their page is blocked just because there is some upcoming law in the US. Its not like they can pick up the phone and call their representative to affect the matter in any way.