If the user is still on your list, they will appear with a tag under their name.
The unblocking system on Steam is highly functional but has some quirks. Once you unblock a user, they can once again send you friend requests, message you through Steam Friends List chat, and comment on your profile.
Unblocking doesn’t automatically re-add them as a friend. You’ll need to send a new friend request (or they can send you one) after unblocking.
If you are having trouble finding the person you want to unblock in your friend list, you can also access your blocked users list. Here's how:
Some users have reported that if you have a massive amount of blocked users, the list might fail to load properly in standard view, requiring Big Picture Mode as a workaround. How to Unblock a User