You're assuming the leak was accidental, the person knows about it, and they didn't intend for others to use it.

What happens when someone sets up a marketplace where people can sell those blind signatures using their ID for $2 each? And then kids just pay $2 to have someone else blindly use their ID to validate the account, because supposedly the system is structured so that nobody can tell which ID was used or tie it back to the account?

That's where the google play integrity / attestation comes into the effect.

In theory you cannot export your private key from the device (from the secure element), so for each $2 someone would have to quickly unlock their phone, scan code via the app and so on.