So obviously I play alot of leblanc
and like the champion alot. I think what has recently happened to Ahri 
is what is happening with leblanc.
Alot of you probably remember the silence removal and the overall
unpopularity of leblanc after the said nerf. I have played her before and then and have stuck with her for awhile now and
will still play her after these nerfs.
I believe it was patch 4.20 but they basically buffed leblancs distortion return timer. Raising it from 3 to 4 seconds. Which
in a game where 3 second cc is considered godlike and the ultimate move of most champions really obnoxious.
Leblanc was basically in a state where she could get 'cced' AND still ESCAPE.
this is rather dumb and lacking in counterplay. since no ones cc really extends over 4 second.
I used to consider annie a major counter of mine but now if i distortion in and get tibbers
I can still most likely immediately distortion back after the cc ends.
In fact my low cooldown passive means I get a vayne period of invisibilty and immediately blink back.
Leblanc is at a point where she can do alot more risky plays without being countered by cc.
This timer on her distortion and the relative low cooldown of her passive are the main problems with leblanc from
a balance point of view.
Now taking my example from Ahri We all remember the time before she had a movement buff on her q.
Ahri was in a fine state but wasnt the highly contested midlaner she is today. Riot's balanced team then
immediately decided to keep the new part of her kit but nerf her with both range and mana costs in patch
5.3
Ahri suddenly got this new mobility mechanic that made her extremely safe and let her dominate or survive bad minlane matchups
and riot forced it on her and made the rest of her alot worse. Ahri as a whole basicaly got elised. Her kit got a bit more overloaded
so riot instead of just reversing it messed her numbers up. a really roundabout bad way fo dealing with the obvious problem.
Leblancs problem in my eyes will never be fixed by a damage nerf. Thats just attacking the wrong problem in her kit. She has a
47% winrate in soloq and 44% right now in competitive.
(http://www.goldper10.com/article/1035-power-picks-february-2015.html/)
(http://www.lolking.net/champions/leblanc#statistics)
Nerfing her damage doesnt negate why shes good right now as opposed to the period where midlaners like bjergsen considered her bad after the silence nerf.
She got a mobility boost. Its that simply. If anything In a perfect world where Riot asked me to balance leblanc, I would simply undo the 4 second buff on her distortion. By focusing on her damage she will be in a state like elise is. A worthless champion who cant win a game or who just got balanced poorly, which i think we can all agree is pretty shitty. Focus needs to be on both increasing her passive cooldown and decreasing the cooldown on the blink back of distortion. She will return to the exact same state she was before and have the exact same kit. Which was considered universally bad at the time. Honestly Its an easy fix I just dont understand why riot isnt doing it.
Obviously I am biased because I love playing her, I play her top, mid support and even jungle, but number nerfs are riots way of trying
to cram mobility into this game alot like with the recent introduction of kalista or when yasuo came out. It just creates problems you can see the clear answer too.
TL;DR
Return Distortion to a 3 second jump timer or possibly lower like 2.5 to make her actually have to RISK something to go in.
Increase Passive cooldown since it makes her at a high level unkillable with the 4 second blink back.
Messing with her numbers and damage will just make her elise. A highly awesome kit thats basically useless like your currently doing to kassadin and how you seem to feel about Ahri.
These are the ways to most optimally balance her in my opinion. give her risk. its simple. Riot is just avoiding that by messing with numbers.