Hi guys! This is my first build and i choose Leblanc because this is a champ i especially like:
When she was new i didn't like her so much but i recently bought her and she fast became one of my favorite champ.

For runes, i take Greater Quintessence of Insight or of Fortitude (this choice depends of your playing way) Greater mark of insight too, Greater glyph of replenishment (this will alow you to stay longer in your lane) and Greater seal of potency for a better harassment power in early game.

I go for a 21/0/9 mastery page, like a lot of mages may take, but you can take a 9/0/21 page too.

The ult is to up at level 6,11 and 16, so as soon as possible.
I choose to up sigil of silence first because this is your main source of damage and harassment.
I up Distortion in second because this skill allows you to farm fast and to escape ganks, also initiating the combo Distortion->Sigil of Silence->Ulti.
Etheral chains is not a bad skill, but it really becomes interesting in mid and late game so i choose to up it in last, but it is also good to get a point in it in early game.

I choose to take Flash and Ignite:
Flash helps you escaping dangerous fights or following low life ennemies. It can make a nice combo with your distortion.
Ignite can make you kill a champ escaping with no life or to annihilate powerfull ultis like Mundo's or Swain's.

Core build:

Doran's Ring -> good to start the game for a mage like Leblanc.
Sorcerer's Shoes -> Just obligatory for magic champs.
Rabadon's Deathcap -> One of the best items for magic champs, it allows you to have powerfull skills.
Archangel's Staff -> Another good item, but you can leave it to a Zohnya's hourglass or another ap item.
Morello's evil tome -> A good item, CD reduction, ap, but if your team seem to dominate the game buy a Mejai's soulstealer instead.
Will of the ancients -> a good item, but if there's not magic champ in your team take another item.

Situationnal items:

Abyssal scepter -> If there's much ap champs in ennemy team or magic resist stackers.
Moonflair spellblade -> A good ap boost and tenacity, take it if there's many stuns in ennemy team
Void Staff -> if the ennemy team has too much magic resist
Zhonya's hourglass -> This item allows you a good ap boost and armor, take it if the ennemy team has too much ad guys
Banshee's Veil -> A survival item, if you fell too squishy or mana dependant, this item will help you, the magic resist and the passive can help you during team fights.

My build is ended, thanks for reading it, and let me a comment if you have an observation or a question.