Could you please pm me the cliet's name so I can try to replicate and advise? You can also send me a screenshot of the steps you're taking when trying to unblock this client.
Click on the message tab and type in the name of the client that you wish to unblock on the search box. Then click on his profile picture, you will see three dots at the right side portion, click on "Unblock". Hope this helps.
Unfortunately, there isn't an option to only view blocked freelancer. It's possible to find them either by searching by their name or by going to View Rooms by Category > People> View All People and then browsing through conversations.