example.com generates a content bundle and signs it. Google.com downloads the bundle and decides to mirror it from their domain. Your browser downloads the bundle from google.com, and verifies that the signature comes from example.com. Your browser is now confident that the content did originate from example.com, and so can freely say that the "canonical URL" for the content is example.com.

Malicious.org does the same thing, and the browser spots that malicious.org is blocked. At this point it doesn't matter if the content came from google.com, because the browser knows that the content is signed by malicious.org and so it originated from there.

Hope this helps clarify. Obviously blacklisting isn't a great security mechanism; my point is just that signed exchanges don't really open any NEW vectors for attack.