I believe blocking an instance hides posts from your feeds but nothing else, but it’s worth testing.
I have lemmit.online (reddit copy) blocked, but I can still search for a specific post and view it. I have also seen others complaining that when they bad an instance they still see comments from users on that instance, so at least at the moment it seems it just hides the posts from your feeds.
I know it’s called Photon because of all the times I got it wrong. Easy mistake to make!