Is a "dark identity" one that's never been shared with an identity-theft-as-a-service? Or is it just of one that's (supposed to be) privacy-conscious (and wouldn't otherwise have been an easy victim)?

Basically exactly my point. If you could use the service without ID verification, and others can still use the service without ID verification, but you've been blocked because you haven't handed over your ID, then leave or start a new account. That is if you're averse to being banned personally. If you don't mind the risk then you can verify ID and prepare to jump ship if it's a ban.