They may have blocked you after you played them
Why can you be paired in the pool against people that you blocked?

They may have blocked you after you played them
They had already blocked me before, but I was paired with them again.

He didn't block you, he was probably temporarily blocked for inappropriate behavior and restrictions were placed on his account.

No, I've been consistently blocked for an extended period of time, but still was paired up with him in the pool.
In my bullet rounds against the pool I was paired up against a player whose handle I won't share here. I had played against them a couple of times before, and when I was checking our mutual score after our previous game I saw that they had blocked me. I've never exchanged any words with them, but it's their right to block whoever they want. So I'm asking this on their behalf (can't exactly say "I'm asking this for a friend", but you get the gist).
The Support FAQ indicates the following:
What happens when you block someone?
Once you block a member, they will no longer be able to:
If this , we're not supposed to be paired up against one another in the pool. And yet, there we were.
Can anyone shed light on what happened here, and what can be done to ensure that my opponent will no longer be paired up with players that he doesn't want to play with?