Oh when you only have 1 orb, casting mind blast will simply consume it. But if you have 2 orbs, casting mind blast will only consume one of them, and with 3 orbs, casting mind blast will still consume only 1 of them, while dealing the same damage as a 2 orb and 3 orb mind blast respectively.
Now that I think about it...the whole idea does sound a bit OP in the sense that you can pretty much guarantee a 100% empowered shadows uptime.