Ok heavy spoilers, but this popped into my head and I had to get it out. The Blackstaff is known to protect the user from the corrupting effects of breaking the laws of magic. And the laws of magic weren't just voted on they are fundamental laws that you do not mess with.

We also are pretty sure that the staff was stolen from Mother Winter. (I do wonder if she stole it from someone else but that is another thread) We also more recently learned what Winter's purpose is in the grand scheme. The Lady gets the solders, the Queen commands the armies, but what does the Mother do?

One of the last, and maybe the most important, laws of magic is do not talk to the outsiders. We could clearly see that dealing with them corrupted even Mauve to the point Mab had to have her killed.

However the Blackstaff allows you to break the laws of magic without staining your soul/psyche. If Mother Winter had the Blackstaff for a significant amount of time it could be that her role, her mantle's duty, is to negotiate with the Outsiders. In my theory she is supposed to be trying to keep the peace with entities from outside our reality.

Furthermore without being able to have negotiations with Mother Winter for so long, the Outsiders are kinda desperate for someone to talk to. This has caused them to redouble efforts to cross over in anyway possible causing the main overarching plot that Harry has been dealing with.

Just my thoughts, would love to hear if anyone thinks I might have gotten it right.