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> you present the same unblinded signature to both services

You would never do this as it defeats the entire purpose of using blind signatures to begin with.

That's the point. You go to example.com and get the "sign in with Google" box as the only login option, but now you can't have separate uncorrelated Google accounts. Or if browsers do it automatically then every site does a background load or redirect through adtracker.nsa so you're presenting the same token on every service.

It's not the user who wants any of this to begin with. "You would never do that" except that it's now the only way to be let into the service.

If A adopts a Blind Signature scheme it implies A is cooperating in establishing privacy infrastructure. If A is so malicious that it would advertise a sound privacy system and then it immediately sabotages it that's a different matter...

The proposals are generally to have the government do the blind signature scheme. But then even if they do so in good faith, the ad services will immediately set to work thwarting it.

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