I'd make the Blackstaff Magic a [-2] that required you to already have Evocation and Thaumaturgy.
My take on Blackstaff Sponsored Magic...
Blackstaff Magic [-2]Musts: Blackstaff Sponsored Magic requires you to already possess Evocation and Thaumaturgy, thus there is no cost rebate for already having these powers.
Sponsor: The Blackstaff, an item of immense power. This power is thought to be ultimately derived from Winter, but it remains distinct from Unseelie Magic.
Agenda: Death, destruction, corruption/transformation, decay, disruption.
Evocation: Blackstaff Magic aids with evocations that break the Laws of Magic, generally in destructive ways.
Thaumaturgy: Blackstaff Magic aids with thaumaturgy that breaks the Laws of Magic, generally in destructive ways.
Evothaum: Blackstaff Magic allows entropomancy and rituals intended to kill mortals to be cast as evothaum.
Extra Benefits: Blackstaff Magic adds a +1 control and power/complexity bonus to any magic intended to kill a mortal. Furthermore, you can take 1 physical stress to increase these bonuses to +2 for one spell.
The reasons I set it up this way:
-The very strong implication the Blackstaff is Mother Winter's walking stick, and Mother Winter's association with death, destruction, entropy
-Jim's suggesting that the Blackstaff is specifically for killing, perhaps more so than other Law-breaking (the WoJ about 'does the Blackstaff have any powers related to the dead', the response was 'other than making people dead? that's kind of the point')
-The Physical Stress bit is based on the 'tendrils' feeding on Eb after he did the big killing spell in Changes
EDIT: incomplete sentence, bolding, evothaum