To view blocked instances if you’re browsing Lemmy on the desktop website go to lemmy.world/instances and click the “Blocked Instances” tab.

Unfortunately there no standard way to view blocked communities, I’m not sure if lemmy.world admins maintain a list somewhere.

I’ll add that the blocked stuff is good/bad depending on your point of view. e.g. some instances (including lemmy.world) participate in Fediseer which helps determine spammy intances to potentially block and keep spam out. That sort of blocking is okay, the other types of blocking I’m not too onboard with but that’s just IMO.