Power (Daily): Immediate Reaction. Use this power when an enemy makes a melee attack against you. Make a melee basic attack against that enemy, with a power bonus on your attack roll equal to this weapon's enhancement bonus; if your result exceeds that of the attack roll against you, the enemy's attack misses. The melee basic attack you make to block your enemy's attack has no other effect and does not deal damage.
Nowhere does it mention specifically that the attack must be made with this weapon, as many other weapon powers do. Can you hold it in your off hand and make the attack with your main hand? Can you use a ki focus to determine your enhancement bonus for the attack? Can you drop it and draw another weapon as a free action (see Battle Harness Armor) to make the MBA with?
I would say that it would have to be the weapon with the power on it.
If you are holding a flaming sword and you attack with a plain sword in your offhand would it deal fire damage?
But, as always, "it's up to the DM!"