How would that even work?
Poorly, would be my guess.
Like, imagine if you block the OP of a megathread. Then you can't read the OP, end up violating the rules in the first post that the title orders you to read, and get yourself banned.
Or less extreme then it can lead to threads either not showing up or you being unable to see the OP.
Also, I presume that given they ask for a block when ignore exists that means they want the blocked user to be unable to read their posts too, like on Twitter.
Which means that if the OP of a megsthread blocks you then jf you want to discuss that issue you'd be forced to keep opening new threads and ask mods to merge them with the megathread once the topic you're discussing is over.
Or, if the OP of say the UK Politics Thread blocks one of the regulars then you'll essentially have two, concurrent UK politics as a result, or the first just gets retired.
In all cases, absolute mayhem.
And you'd be unable to see if somebody had made a thread about what you want to discuss, if they've blocked you, so you'll make a duplicate.
And if the dupe is merged with the original you can't see it anymore as OP blocked you, so you're forced to make a dupe...
Again, absolute mayhem.
Perhaps there's ways to do it to avoid mayhem, but I don't really see how you'd do it.
Even just hiding non OP posts the person who blocked you made can cause mayhem too, as it can lead to multiple people repeatedly posting the exact same stuff when new stuff happens, leading to lots of thread bloat.