This sounds like it would be a good underpinning for a decentralized blockchain file storage system with its focus on immutability and redundancy.

But a blockchain is already immutable. It becomes decentralised and redundant if you have multiple nodes sharing blocks.

No need for an underpinning, it is the underpinning.

if you're storing the blocks in one place, its not decentralised.

The metadata would be crucial for performance, and given that I assume you'll want a full chain of history for every file, your metadata table will get progressively bigger every time you do any kind of metadata operation.

Plus you can only have one person write metadata at one time, so you're gonna get huge top of line blocking.

And yet no one needed a blockchain to implement this.