If you've forgotten the Gamertag and they aren't in your "Recent Players" list, you may need to check your settings via a web browser: Go to Xbox.com and sign in. Go to your Profile > Privacy Settings . Navigate to the Xbox Online Safety tab.
Blocking someone automatically removes them from your Friends list. Unblocking them does not automatically add them back. You will need to send a new friend request if you want to become "Friends" again. Can’t Find the Person You Blocked? how to unblock on xbox