Here's the thing I don't get about the Blackstaff:

In the RPG or maybe a WoJ, the thing most special about the Blackstaff is that it allows its bearer to do black magic without it staining their soul. Just sucks it right out of them.

If that's the case, what does Mother Winter need it for? Or, is that simply what it does in the hands of a mortal?

I agree that the stone table does share elements with the recurring notion of 'ascension' but it let's not forget it also shares elements with the idea of balance insofar as it plays a pivotal role when the seasons change. It's also a ritual item too.

Consequently, this overlap plus its own distinctions and that it occurred in more than one book make me think it could have its own entry.

Summer Knight also introduced:

Titania
Summer Court
Lily
Fix
Mother Winter
Mother Summer
Stone Table
Lloyd Slate
Concept of constant balance between courts