I would love to add this, but I’m hesitant at the moment as just adding this as an Alexandrite feature is only a half-fix. I’d just end up not showing posts for that instance, but Lemmy would still send all of the posts to Alexandrite regardless, so it’d be possible if you blocked enough instances that Alexandrite would have to load 5 pages of posts to show you one page of results (or worse), which could add extra load on the servers that wouldn’t happen if it was officially supported by Lemmy.

It’s a really popular feature request though, so I wouldn’t be surprised if it gets added to Lemmy in the (hopefully not too distant) future, after which I will for sure implement it in Alexandrite. Relevant Lemmy Github issue

I’m not sure about Alexandrite or blocking instances, but you could consider switching to an instance that defederates instances you don’t like. Lemmy.world is pretty relaxed (pron allowed etc)