No. Facebook does not send a notification to the other person when you unblock them. They will only realize it if they search for your profile and find that it is visible again, or if they try to message you and it goes through.
Note: Unblocking doesn't automatically restore past messages or re-add them as a friend on Facebook — you may need to send a new friend request separately.
Note: This will unblock them on both Messenger and Facebook simultaneously.
Tap the icon (three lines) and then the Settings (gear) icon. Select Privacy & safety . Tap Blocked accounts . Select the person’s name from the list.
That's it! Once you've unblocked someone, you'll be able to see their messages and interact with them on Messenger again.