Never saw a prompt or other mechanism to explicitly unblock the user - as far as I can remember. They don’t appear at /admin/users/list/blocked, and Blocked is false in the admin page for that user.
However, I do see at /admin/logs/staff_action_logs that I have apparently unblocked the user a day ago. I don’t recall doing that though - can unblocking be a side effect of some other admin operation?