[mikeash]

The Bitcoin blockchain solves the problem of maintaining a distributed public ledger such that changes are agreed upon by all and can't be changed retroactively.

You don't need any of that if you just want a torrent index. There's no need for consensus and no need to keep a secure record of the past. Bitcoin needs to prevent Joe from forging messages like, "Bob sent Joe one thousand bitcoins," but there's no corresponding scenario for torrents.