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.