The problem with Hidden services/ TOR isn't search. It's the fact there aren't enough legitimate/trusted websites that appeal to users outside the hardcore privacy/security crowd.
edit: fixed Tor capitalization.
Though writing this gave me an idea: is there a .onion Torrent index?
edit: fixed Tor capitalization
Beyond that, I think that dedicated TOR users will shy away from using BitTorrent since it's a huge privacy risk. They're more likely to be using FreeNet or I2P for anonymous P2P filesharing.
It is The Pirate Bay
Also, it's Tor not TOR. https://www.torproject.org/docs/faq.html.en#WhyCalledTor