How to unblock someone on Facebook
Go to facebook blocking page by clicking the direct link below:
https://www.facebook.com/settings?tab=blocking
Then look at Block Users, you will see the list of Friends you have blocked.
Click the x next to name and unblock the user you want to unblock on Facebook.
Remember, once you block user, they are automatically removed from your friends list. So if you want that user to be your friend again, you will be needed to send the friend request to him/her