Unblocking someone also unblocks them from sending you app invites and game requests, so be prepared for those notifications if they are an avid gamer.
You unblock an ex. You immediately go to a mutual friend's page to see if they commented on a photo. Why? Facebook caches blocks. It can take up to 15 minutes for the unblock to propagate across Facebook's servers. Close the app, take a deep breath, and try again in ten minutes. how to unblock somebody on facebook