If you accidentally block a host, there are two methods to unblock:
Go to the search page, enter the host's account, and after entering the host's profile, click on the "..." in the top right corner. Then, click on "Unblock."
If you forget the host's account, click on the account icon in the top right corner > go to the profile or settings > manage profile > block list. Find the host you want to unblock and click on "Unblock" on the right side.