The most direct way to unblock a user is through your Friends List inside the Steam application. Launch your Steam desktop client and log in.
Alternatively, if you know the user's profile URL, you can navigate directly to their page, click the dropdown menu near the top right of their profile, and select "Unblock all communication" . How to Unblock Someone on Steam (Mobile App) how to unblock someone steam
Look for the "Blocked" tab within your friends list. The most direct way to unblock a user