Is it possible to destroy a Party Upgrade Card, such as Party Boat, with crushing blow?

Crushing Blow does not mention Party Upgrade Cards for obvious reasons: the card was introduced before RtL and Party Upgrade Cards are not used in the normal game. Crushing Blow allows OL to destroy an item that is carried by a hero. Party Upgrade Cards are items (p 4) However, are they carried for the purposes of CB?

I am inclined to say no because: 1) CB is the best treachery card and it would make it even better (some Party Upgrade Cards are very expensive) 2) no particular hero carries these items.

No, you can't CB a party upgrade. Or, at least, you shouldn't be able to. The points you make are probably the best arguments against the idea.

You most definitely cannot destroy party upgrades with Crushing Blow because nobody is carrying it.

+1 to both Steve and James`es answer.