But I know an admin can ban or block communities from other instances
I am pretty sure this is incorrect. Admins can not block select specific communities from other instances. Federation is by whole instance or nothing.
Every instance maintains a public instance list which shows which other instances are federated and which are blocked. For exampke, here's the Lemmy.World instance manifest: https://lemmy.world/instances
When instance-a blocks (defederated from) instance-b, it won't show up in any modlog, it will show up on the instsnce list.