natoal The GrapheneOS site said it's possible to obtain the keys...
For sure it is possible. At least several nation-state actors could do it. That doesn't mean it's easy, and it has been designed to be hard.
natoal If an app like Netguard was used that blocked google domains, that might prevent them from updating the blacklist of keys.
Possibly, but that wouldn't stop the server side of the app from noticing that the chain included a revoked certificate.
natoal Of course, a key only used to download one or two apps might not even be flagged. Could a new key be generated after one is flagged?
I suspect a good model for this is that extracting a device private key costs somewhere between $10,000 and $10,000,000. Clearly Google hopes it's expensive and has worked hard on that. I think it will be a surprise if key extraction is something like $10. So using an extracted key to download a small number of apps may not be efficient, and spreading the cost across many users seems likely to result in discovery and key rotation.