I thought I saw on a promo video for the Un'goro expansion for Hearthstone that it was possible to use the Un'goro Rogue card "Envenom Weapon" in combination with the classic Rogue card "Blade Flurry" to hit all enemy minions with a poison attack at once, wiping them out. When I tried it, it didn't work. Is this not possible or did I not set up the combo right?
Envenom Weapon does not make Blade Flurry poisonous. As stated in this article,
Envenom Weapon's Poisonous ability does not carry over to the effect of Blade Flurry.
It references these two tweets, which explains how blade flurry works similar to using shadowflame on a minion with poisonous. It is the spell that is dealing the damage, not the weapon itself, so the poisonous effect does not carry over.