Depends on your perspective.
On the one hand, it does not actually let you do anything at all that you could not already do (other than summon a really cool stick made of black smoke, I guess); there is no power boost or additional special abilities granted.
On the Other Hand, it lets you do ALL THE BLACK MAGIC without having to worry about turning your psyche into swiss cheese from what amount to the spiritual equivalent of Brain Damage. If you're already down that swiss cheese road it's probably not going to give you much you didnt already have. Im not sure it would have any appeal to your Kemmlers or Cowls, for example.
I wonder if, say, Kemmler picked up the Blackstaff, would it automagically absorb all the blackmagic taint he's accumulated over the years, or only protect him from more taint when he used the Blackstaff.
Now, assuming it absorbs it all, I do not think it would heal any damage done through the use of Black Magic. What I mean is that if black magic twists your mind, your mind will remain twisted after the influence is gone, you just wouldn't become any more twisted.
