I have a quick question regarding the interaction between these two cards.

Let's say you take control of Venomous Blade and attach it to your own character. You lose a challange and send it into shadows.

Are you now allowed, as a Lannister player to bring it out of shadows and re-use?

(assuming I don't have the Shadows agenda)

yes. house Martell only is only a check for deckbuilding, not in game use after a steal. Its the same for a seductive promise of a house X only character.

Lars is right that in general, you are allowed to take control of - and use - "House X only" cards from another House during a game. There is a larger consideration specifically for Venomous Blade, though.

Whenever a card leaves play, for any reason, all "take control" effects are eliminated and they are returned to their owners. You know how when you take control of a character with Seductive Promise, it goes to its owner's dead pile when it is killed or to its owner's hand if it is returned to hand? Same thing here. If you take control of Venomous Blade and return it to Shadows, it goes to its owner's Shadows area - where he could bring it out of Shadows all over again.

So the real combination of Compelled by the Rock and Venomous Blade is to take the attachment away from the Martell player and keep it out of his/her reach. Trying to use it will just end up giving it back to them.

Thanks for the quick answers ktom and lars.