How about if the owner of the blockchain (here I'm assuming, say, a land-ownership database run by the state) have a private key for each person in their own sy

How about if the owner of the blockchain (here I'm assuming, say, a land-ownership database run by the state) have a private key for each person in their own system which they use to put the relevant data into a blockchain. Then they delete the private key for that person to scramble entries that need deleting so no one can ever access it again. The person doesn't need to have or know their own private key since it's a backend for the municipal service. Now why anyone would want to do this with a blockchain instead of a simple encrypted database... well, maybe someone else can explain that. I'm still assuming you visit deeds.mass.gov or something and have an interface you would use to find out who owns what, so the end user isn't actually touching the blockchain in this case.