How would users know that tokens had actually been signed blindly?

If the tokens are never sent, only their blinded versions, it is pretty much guaranteed that the signature you get back was made without looking at the actual token.

I get that. What I wonder is who would nontechnical users need to trust about that? CloudFlare? The Tor Project?

I'm not sure, but it can be done with just CloudFlare changes; if the plugin is open source it should be fine. Maybe if Tor Browser integrates the plugin it should be fine too.

Optimal would be only needing to trust the Tor Project.