About 15 months ago, Google introduced a block results feature in the search results user interface. That feature went missing when Google launched Search Plus Your World earlier this year.
Well, it only went missing for those who use Google+ when searching. For those not searching using Google+, the feature remained. I assume that is most of the Google user.
But now, when you block results in Google's search results, you can't unblock it. I tried it myself and you cannot unblock results. I blocked a site, went to the manage blocked sites and clicked the "unblock" link and it did not work.
Google has yet to confirm the bug and respond with an estimated time for a fix. I assume they will be in no rush to fix this but we will see how easy it is for them to push out a fix.
Forum discussion at Google Web Search Help.