> Even more specifically, a blockchain is for when you want Byzantine fault tolerance, i.e. you don't trust one or more of the actors involved.

Often yes. But also blockchain's can be useful simply for backups and scaling: by cryptographically linking every bit of data together you can be confident that you actually have a complete copy without any errors.

Git is basically a blockchain for this exact reason: starting from a git commit hash, git works backwards, checking that every byte of data is what it should be. Similarly, modern filesystems like btrfs use strong (if not cryptographically strong) hashes for this same reason.

Though in a sense, you're still correct: the "actor" you aren't trusting here is your own computer hardware.

I think you're technically correct here: if you just have a bunch of Merkle trees where each one tracks the hash of the previous block, it would be accurate to refer to it as a blockchain even if you're not bothering to implement any of the distributed consensus algorithms that cryptocurrencies are actually known for. It's probably not the first thing that would come to mind, but it is a correct way to use that word.