"I say we forget this business and run."
A good catch, and one I hadn't thought of.I had a thought and just tested it.
I discovered you can't add somebody who's blocking you to your own block list.
Given the creation of new sections which can only be read by logged-in users, this asymmetry could be exploited to allow people to sneakily read the posts of users they block, by temporarily unblocking them while they snoop.
One possible - if heavy handed - solution may be to see if you can add a timer to how frequently you can Block / Unblock somebody? I have no idea if that'd work with Xenforo's software, but if staff added something like a 24 hour, 12 hour, or even 4-8 hour timer before you could (un)block somebody added or removed from your list that would likely do the trick. If you're using the system as intended it most probably won't impact you in any noticeable fashion (the worst that can happen is you accidentally press the wrong button and either just wait half a day or PM a moderator asking to undo the hiccup), meanwhile if you're exploiting the system it'll be incredibly obvious when every other half-day somebody's posts reappear or disappear while viewing the forums.