Does the Rhythm Blade [ddi] bonus to AC and Reflex work when you've not got an existing shield?
Property: While you wield this weapon in your off hand, your shield bonus to AC and Reflex increases by 1.
I've seen it argued both ways, but if it works without an existing shield, why would you ever go for the Shielding Blade [ddi]
Property: You gain a +1 shield bonus to AC.
which is a Level 4 magic item, as opposed to the Rhythm Blade Level 3
Best Answer
The Rythm blade increases your shield bonus, the other blade provides one. This makes it's quite clear in my reading that your existing shield bonus (+0 if you don't have one) is increased by one, allowing it to stack with your existing shield or other shield bonus. However, the Shielding blade provides a shield bonus which does not stack with a shield or feat/class feature/power that grants a shield bonus.
The level difference can be accounted for in the following ways:
Rhythm blade requires being wielded off hand. Shielding blade has no such requirement.
Rhythm blade is restricted to light blade. Shielding blade can be heavy or light.
Pro tip: Wield both of these to get a +2 AC and +1 to Ref!