Another question is: why would you need it for a key?

Long-established public/private keys and signatures are used in this way all the time to control access to servers around the world. No blockchain needed. Blockchain is helpful when we all need to agree on a series of events.

Homes are a nice example of where you can have an isolated system which knows what it needs to about you (e.g. a public key) without sharing or cross-checking anything with the world.

Blessed be the fruit.