It doesn’t ensure privacy as such, it does close to the opposite by design.

Blockchain creates a public record (distributed shared ledger) with state changes signed by private keys.

Anyone with access to the chain can then determine if a change was signed by a private key with the right to do so. The integrity of the blockchain is protected by proof-of-work or proof-of-stake.

Where this can promote privacy is that if the knowledge of who owns / controls a private key is secret - you can’t know who in the real world is behind it and any value associated with that key unless someone chooses to reveal their public key.

Implementations such as Bitcoin are also completely decentralised and autonomous so there is no person or organisation with any kind of privileged access. By consensus the community can role out breaking changes and even alter the chain - as Ethereum did following the DAO attack (to return $64m of value that was stolen through a software fault) though.