LifeLites has just announced a blue LED lightsaber for your minifig duals. The saber and blade are all one piece with a 6 inch cord coming out the bottom of the hilt. The accessory itself is $8 USD, and you will also need to purchase a separate power source. The good news is that it's compatible with a wide variety of LifeLites kits.
Supplies are limited, so act soon. Hopefully if these are successful, other color blades will be made.