Unblocking isn’t done inside a specific conversation. Instead, you have to go to your device’s (or the Phone/Messages app’s block list) and remove the contact from a blocked list. You cannot simply swipe or tap “unblock” from within the chat thread itself.
: Select the conversation with the person you wish to unblock. how to unblock someone on messages
A simple reboot can refresh your carrier settings and sync your blocked list. Unblocking isn’t done inside a specific conversation