I think a whitelist is worse. While it gives them more control, it also essentially shuts out smaller instances and can lead to the existing instances becoming entrenched and hard to replace. I'm also not sure if would be a good move 'politically' for the admins. People are going to treat whatever is whitelisted as being endorsed by them.

Allowing instances to use a whitelist instead of a blacklist is actually a really bad idea. It makes the default not being federated with anything, which makes it far easier to create centralized isolated silos that it's hard to move off of while staying in contact with, and in general would just destroy the interconnected nature of the fediverse