I was just meaning to ask this. I have a list of 42 communities I'm subscribed to on this account that I can't add to my pornlemmy account, and I'm sure there are more. Some of them are from lemmynsfw, some lemmit (but others work fine), some lemmy.world, etc.

Is there a reason some communities from federated instances are not available here?

Yes, they weren't in line with what I want pornlemmy to represent.

Can individual communities on another instance by blocked

Yes, but unfortunately

is it possible which communities are?

as far as I know, there is no way for the users to get a list of blocked communities.