I believe your explanation but it still strikes me as the wrong thing to do.

I'd like to propose "Zero Access", as in zero access to the plaintext.

That's actually not bad. I was fine with SpiderOak going for a term that is simple, catchy, and easy to market. Zero Access is the kind of alternative that might work. That specific one might have a problem: send perception of user having zero access to their own data when most clouds constantly reinforce "access from anywhere any time."

You're thinking along the right lines. I think variations of the words safe and vault have worked for other companies, too, given people understand what they do. "Your data is in a locked vault that we hold for you while you keep the keys or combination." That sort of thing.

They could go with "Know nothing".

(Or "Jon Snow", for short, if they can get away with it without being sued - "Jon S. crypto, we dont know nuthing" ;-)

This doesn't feel like the correct solution.