Blockchain could one day solvethe online privacy problem by encrypting or scrambling personally identifiable information and issuing each person a random string of bits & a private key & created explicitly for unscrambling their data.The person holding the blockchain private key could issue various public keys controlling who has access to the personal data on the blockchain.
So, for instance, if a car rental agency needed to verify you have a driver's license, you could use a public key to give them access to that information.
You could later revoke access to that information.