A person's testimony is a source. I can add mine: you can tell when you're truly blocked because if you click for the accessibility audio-based captcha it will actually tell you you're blocked (but, if you did the visual captcha, would simply loop forever while telling you you did it wrong).

I don't think you'll find an article by Google saying "yes, we sometimes completely block users while making it look like they're not blocked and wasting their time".