A class action is an action where there are a large, possibly unknown, number of plaintiffs suing a single group of named defendants. With the thought process being that if a single case against a named defendant is won, then that same outcome would happen for every other plaintiff, and thus doing it once saves the court time.

What you're suggesting is more like a John Doe case where you sue a number of unknown entities, and that can be won, but at some point, the plaintiff has to name the John Does, so that they can defend themselves.

Couldn't you start a class action against one named defendant, and use the discovery process to uncover all the other unknown-at-the-time-of-filing defendants?

I'm pretty sure that if the named defendant coughs up an NDA that prevents them from disclosing the names of their business associates to a court, the judge is not just going to say, "I'll allow it."

It has successfully been done in the past, so yes, you could do that.