Or even a signed hash against a confirmed/known/distributed public key. Blockchain is only useful in a decentralized + adversarial context where there is a nee

Or even a signed hash against a confirmed/known/distributed public key. Blockchain is only useful in a decentralized + adversarial context where there is a need to remove one-another's ability to game a given system. And even then, it doesn't imply a perfect security around it. Since it won't stop theft, breach, scams or other vectors.