Firstly: there is no longer any such thing as an always on item.
Like EdgeOfDreams already said, always on items are a remnant from the Early Bird PDF's.
That makes the question of defense more or less moot. Since the Energy blade only exists for Attack purposes for a short moment, there is no way to defend with it.
What Skill: you define the Skill used when creating the Item. A Magical Gun could use Guns to aim, a sword Weapons, a wand with charges of fireballs in it could be using Discipline.
Extending on the Defense question: I would allow such a weapon, that is a physical handle with an energy blade extending from it, to be used for defense, but only is the user has already attacked with the weapon in the current round or expends a charge to defend, which would then allow him to attack if his initiative is still to come. Basically you can only defend if you expend a charge in the current round.
The Skill to be used for defense would be Weapons. Unless you have a stunt that says otherwise.
If you make a physical blade that is magically enhanced, the defense issue disappears. It could be used for defense like any other blade.
I made a stunt like that once
Its was something like
-1 Master Parry (Weapons) You can use Weapons to defend against ranged attacks that could be conceivably be stopped by a melee weapon. Things like bullets, Arrows or other small-enough items. No defense against Energy Attacks or things like Thrown Cars or similar things.
You can easily replace the Weapons skill with anything you like for this stunt.
Nobody cares about Mythbusters here when it comes to stunts :-)