Leblanc is a single target burst AP carry who is chosen for pick compositions and is highly effective at creating 1-man advantage teamfights. She is not effective at sieges due to unsafe waveclear. she also cannot defend or push waves easily compared to almost any other mid. She generally is not effective in 5v5 teamfights unless wildly fed. Shes a high skill cap champion who probably has some of the highest outplay potential.
Morello and NLR are the core items on leblanc as she is pretty useless until the point she can 1-shot an opponent with one full combo. Until she reaches that point often an enemy can back after her burst or just simply catch her with cc since she cannot assasinate.
Morellos simply because it gives more AP than Athenes. Leblanc needs the AP and most defensive stats are wasted on her since her crazy mobility should be how you survive with her. She is one of the squishest champs in the entire game and also probably the most effective user of Guardians Angel
Most pros do forgo full 40% CDR on leblanc, but personally I think its one of the strongest build on her. Your Distortion gets to a ridiculously low cooldown and makes her safety insane. Its impossible even as a kassadin to catch her at that point. At the same time no one can possible escape you 1v1 either. But most Leblancs do not build her in this way simply to maximize her damage which often is not the problem late game.
In pro play W first max, followed by Q and then finally E is the standard set. It increases the amount of time you can proc Q's and decreases the W cooldown extremely early. Also adds the only way to increase waveclear on her. Some people prefer Q max, But the loss in mobility on w makes her easier to kill and limits how often she can proc the Q which results in less damage over time and fewer total combos.
Leblancs Lvl 3 and 6 All-in are one of the strongest midlaner powerspikes. with a flash ignite its very possible to secure a first blood at either time in the game.
In terms of items Morellos is the best point in which she becomes a high roaming threat due to the CDR. She then can distortion and take a much different pathing to roams than other mids and still have her distortions off cooldown when she arrives for the kill.
the obtainment of any NLR and then the completion of the first NLR item are also huge spikes for her. Generally at this point she can 1-shot her lane opponent or be totally sure she can combo to death an ADC
She synergizes really well with Waveclear ADCs since she lacks it herself. Lucian, Sivir and jinx are all great adcs to go with her. Having a scaling member of your team is also important since leblanc will fall off generally in very late game 5v5s. This makes Nasus, Jinx, Kog'maw or Master Yi good possible teammates since she can carry the early to mid game making it possible for these champiosn to reach their power spikes. In terms of Mid-Jungle Synergy she excels with a hard cc Jungler. She can easily stack her chains with a cced target or just simply allow her to hit a full combo. Vi and Amumu go really well with her as they bring enough cc to completely hold down targets for LB to assasinate.
Hard CC lockdown or high Waveclear are her main weaknesses. Lissandra, Annie, Cho'gath have great lockdown and burst combos to deal with her. In terms of Waveclear Malzahar, Xerath, or Morgana are also great choices. Also shielding can be somewhat effective against her if she is not given a lead. Champions like Lulu, Orianna and Morgana are great at disrupting her combo.