What does it mean to block a topic?
You might find that some topics are well “off-topic” - either being spam, irrelevant or just plain not useful. Of course, deciding what is and is not useful for your purposes is in the eye of the beholder. When you do see a topic that you do not want the system to follow, you can block it. Once blocked, the topic is moved to the Blocked list and no longer appears on either New or Saved lists.
Can I unblock a topic?
Yes, you can unblock a topic at any time. You can either save it and keep all it history in the newly created Saved Topics, or you can add to New Topics and just start listening for it again.
Is a blocked topic frozen (e.g. not subject to any new messages or topic drift)?
A blocked topic is active, and collects messages just like Saved Topic. It is just hidden from your view. You can and should check what is inside it every now and again - similarly to reviewing what is inside a computer Trashcan, to be sure nothing useful ended up in there.