Bitcoin lets you store 80 bytes with each transaction. Maybe that isn't enough for a magnet link and description, but you could use multiple transactions. Maybe there is more optimal blockchain design though.
Ideally you'd want a way of validating the content as well.
A hidden service is still centralised though, right? Given the current chasing of The Pirate Bay, I'd expect this hidden service to get the same scrutiny as The Silk Road.
My take on The Silk Road was that whilst it was quite possibly bad operational practice that took it down, it could have yielded to traffic analysis. I'd expect the same fate of Tor Pirate Bay.