Changeable definitely, would have to think about whether default block is the smart thing to do. It’s hard, I really dislike Meta, but on the other hand if you go censoring instances by default, where does it stop?
IMO this should be the job of articles that tell you how to setup Lemmy, mentioning something like “Note that by default you also allow Meta the access to your instance which might mean privacy breach, here’s how you disable it.”