I don't have the stats I used handy; but I set up the Black Staff as a bound Outsider. It allows for the holder to avoid Lawbreaker status by the wielder feeding the part of his soul that is tainted by the use of darker magic to the Outsider as payment for the bonuses to the casting. Essentially, Sponsored Magic with the price/debt being a chunk of your soul. The black veins, in my version, was a physical manifestation of the payment/feeding after use which is modeled by taking Stress; pick your track. Even social. One stress for every spell cast in which the Staff was used at the end of the scene. For every Stress "fed" to the Staff, you can remove one Lawbreaker level. This can go right into Consequences; in which case any consequences taken are flavored by the Staff. Oh, and be careful getting down to an Extreme Consequence or, worse, Taken Out.
It also requires one to have the Aspect "Black Staff" (or some variation) which can be compelled left, right and center to bust out the staff to make the character's life so much easier. Any Fate points spent to invoke this aspect goes to the Staff.
It also gave some refinement bonuses to casting and all that, of course. The big thing is that the staff WILL collect payment at the end of the scene in which it is used. So, using it in a fight can be a real gamble. Plus, those compels on use are going to come down hard and fast when you are already down and hurting. The Outsider bound inside the staff wants to consume you; period.