I think Darkness is too prevalent to really be marked as a rare and exotic element on the level of Moon and Music, even if it isn't part of the Imperial Eight or Traditional Five.
Darkness is definitely an uncommon, exotic element though? The Darkness cultivators known are all family traditions(Huang Da is a Count family, Meizhen is a Duke family) or rare drops(Renshu and Ling Qi)
Like, Huang Da got a token from simply successfully cultivating Darkness at Red.
As element rarities go, my impression is:
-Common: Imperial Eight, Traditional Five. These are the stuff that are basically omnipresent across the Empire, with the elements in the overlap(Fire, Water, Earth) being particularly common. It'd be rarer to find someone without SOME proficiency in them.
-Uncommon: First order derivatives. Light, Darkness, Music, Cold, etc. These are things that you
usually can reach through insight in a Common element. Heaven or Fire to Light, Earth or Water to Darkness, Wind or Thunder to Music, Wind or Water to Cold. Certainly you could go straight to them, but its going to be tough. These guys can be found in any given batch, but they will not be common in any given batch.
-Rare: Second order derivatives or unusual and demanding elements. Moon, Sun, Life, Death, etc. These are stuff which are extremely hard to come across, or they will bend your life around the art. You won't expect to find more than a handful of these even in a group of a thousand, unless its a specialized group.
Given the distances involved from the Thousand Lakes and the Emerald Seas, I find it hard to believe that Xiao Fen was instructed to come to the Argent Sect at the last moment and then made it in time to be here for orientation. Additionally, there would be time to register her and pay the fees involved. It's possible that the favor was used to expedite Xiao Fen's travel to the Argent Sect, but I am pretty sure that Xiao Fen coming here was already preplanned for a while.
My belief is that the favor is going to be used to get Meizhen assigned to Cai Rexiang's court as a representative of the Bai. Maybe that won't be needed now with the Bai/Cai alliance, which then the favor is probably going to be held on to for a bit. A favor later is probably going to be more valuable than a favor now.
My belief is that the favor is spent to assign Meizhen as future ambassador to Cai Renxiang, but
such a favor is hardly going to be announced. Not in the Bai!
They'd just send her like they meant to do it that way to begin with.
That said, I believe Xiao Fen was probably dispatched early to capitalize on the alliance and to bond with Meizhen. Likely in the normal course of things, Meizhen would have stayed the mandatory second year in the Sect, then go home to settle down and bond with her Bai-trained handmaiden.
Events changed that:
-Xiao Fen just so happens to be the right age for the Sect. Even in a clan as extensive as the Bai, finding a candidate of the right age to send with enough Talent to not embarrass the clan(by which they mean take the top four) and yet not important enough to be simply kept in-house is hard. But Meizhen's assigned handmaiden is naturally of close age, and at least as expendable as Meizhen.
-They needed someone who could make use of having a Senior in the Sect to advise about this...unfortunately back home Meizhen is a cultivating shut in, so the only peer she has any kind of positive connection with would be her assigned handmaiden.
-Theres probably some cultivation hoodoo for a servitor clan arrangement. Xiao Fen would likely need some access to her primary during her Early Green to get their domains to synchronized properly. Since Meizhen is staying with the Sect and then likely leaving around the same time as Renxiang, the usual solution of doing it at home after a year of Inner Sect is out. So they send Xiao Fen to get into the Inner Sect by the second year to get the magic done.
I expect Xiao Fen learned she was to pack to go to the Sect around the middle of last year.