Crazy WAG Theory 4: The Blackstaff can cure Nemesis corruption.
When Ebenezer uses the Blackstaff at CI, we see those black-tendrils curing him from the corruption. Call me crazy, but those black tendrils remind me a lot of the description we get of Mordite and the Mistfiend at the end of Turn Coat (which Harry reminds us was an Outsider). If the two types of corruption are similar (or identical), then it makes sense that the Blackstaff can cure Nemesis corruption.
For more possible evidence, consider also that the only "onscreen" cure to Nemesis so far is Lea -- cured by the Queen of Winter, who is also in charge of the defense against Outsiders. If the Blackstaff really is Mother Winter's walking stick, then it certainly makes some sense that it would have anti-outsider related powers (similar to the "Unraveling"?). The cure for "normal" magic corruption might just be a side effect...
I'm not sure we have enough information to classify the Blackstaff's mode of operation as more like a "filter" or more like a "cure." If it's a filter, then it would only protect Eb from corruption that he would otherwise gain while wielding the Blackstaff. If it's a cure, then it might well "eat" the diseased parts of Eb's soul that were corrupted even before he picked up the Blackstaff (hypothetically speaking). The visual effect of the black tendrils tends to support the "cure" idea, in my opinion, but we don't
know exactly how it operates.
Crazy WAG Theory 5: The Blackstaff itself was the true target at CI
Another implication is a renewed look at Changes. I personally have never been happy with the notion that the Red Court all got together to throw a super-uber-massive spell just to kill Ebenezer. Sure, he was a powerful wizard, but it still seemed like overkill. If the Blackstaff is really an anti-Outsider weapon I could totally see the BC manipulating the Red Court into using overwhelming force against Ebenezer, knowing that in doing so they achieve the true target of getting the Blackstaff out of commission?
I don't agree with this part, mostly because I think it is incomplete in several respects. First, I think there were at least two or three chessmasters operating on Team Evil's side, including but possibly not limited to the Red King and Nemesis itself. I think the Red King got set up; Nemesis knew there was a chance that the Bloodline Curse could end up backfiring on the Red Court, but it was worth risking the pawn to take out some combination of the forces that threaten its longer-term goals.
In that case, the Blackstaff was certainly
one target of Chichen Itza, but I don't think it was the
only target. Per some of your later points concerning Faith magic, all three Swords of the Cross were also present, each of which is probably at least as significant as the Blackstaff. Both the Winter Knight and Queen Mab's right hand woman were there--the loss of either would not only limit Mab's influence, but also reduce her ability to fend off the threat posed by Maeve. The Grey Council was also drawn onto the battlefield, and while we don't know the Council's full roster, Eb and Odin's participation indicates a heavy-hitting group, in my opinion. In the giant mystic powder keg that was Chichen Itza, any number of significant artifacts or entities that threaten Nemesis might have been taken off the board--the Blackstaff is a noteworthy item on the list, but I don't think it's alone on that list.
