I'd say that a key feature of anything using a 'blockchain' is that there needs to be a way to determine the definitive, actual blockchain. Merely having a cha

I'd say that a key feature of anything using a 'blockchain' is that there needs to be a way to determine the definitive, actual blockchain. Merely having a chain of signed documents is not enough, you also need a mechanism for trusting this chain of documents over that chain of documents. If you don't have this mechanism, or if your use case doesn't need this mechanism, you probably don't need a blockchain at all, and should just be using a database, as it would be far simpler and more efficient. Once you decide that you need some way of having a single global accepted version of your blockchain, you'll have to work out a way of picking one. Proof of work is one of the ways you can do this.