In order to unblock someone on Facebook, a person has to click the "Unblock" button or link next to the profile name. Once clicked, it will allow that person to see the user's profile again.
Clicking on the upper right corner of the homepage will take the user to the option "View All Blocked Users." Confirming the "unblock" dialog box will, however, not add the person back to the friends list automatically. If a user wants to become friends with that person again, he or she will have to send him or her a new friend request. Plus, a user cannot unblock a person until after 48 hours of blocking him or her.