Thanks. I hadn't seen the comment linked to before, https://news.ycombinator.com/item?id=7364465 most informative explanation I've seen.

How do you make the persona brand invisible, while still giving users some visible feedback that they aren't sending their global password to random site x?

Persona didn't have the notion of a global password -- the ideal flow had you authenticating with your email provider, on their domain, which was often completely transparent in cases where you were already signed in to that provider.

Try logging into 123done.org with whatever you want @mockmyid.com to get a sense of what that would ideally look like. My talk from PyCon 2013 goes into a little detail about how the protocol works: https://www.youtube.com/watch?v=nJff23UdNAI

With most email accounts being provided by gmail, outlook, and yahoo, I don't see auth via email domain ever happening.

Why? Gmail is already supporting that.

The question is how do you effectively communicate that to users without establishing a Persona brand?