Any messages they tried to send while blocked are lost. WeChat does not deliver "backlogged" messages once unblocked.
If you blocked deleted the contact, unblocking them only moves them to your "deleted" state. You will need to send a fresh friend request to start chatting again. Can I block someone again immediately? how to unblock friend in wechat
There is a big difference in WeChat between and Deleting a contact. Any messages they tried to send while blocked are lost