i have played roughly 500-600 games with Leblanc.
This is a very specialized build for Leblanc, primarily because i don't get any survivability with her, meaning you have to know how to use her abilities to sustain yourself in a lane. Because of this, i get clarity and teleport, also spell vamp early. i do all of this because she is an assassin, meaning she bursts and leaves, and you need as much AP as you can get to do this effectively. with a proper spell rotation you should be able to kill your target without taking any damage(and if you do, your not picking the right targets). Additionally, this build actually gives just under 900 AP, not 637.
using the build I am almost always the leader for kills in the game, which is the role you want to take with Leblanc.

a very generic build rune-wise. ignoring all but AP and MP, because that's all you really need with her.

again, awfully generic, but it gets the job done.

there is some leeway in this (i.e. if they have a Karthas buy a Zonnya's hourglass) sometimes ill get a lich bane, this is a decent way to play her too. you can buy health potions to start out with, i don't personally. this build will give just under 900 AP, with 30/ 50% spell penetration, which is very important for her.

get distortion first, to avoid early game ganks, also her sigil doesn't reach its potential until you have a second spell to set off the second part. I max sigil first, then distortion, then chains.
it is worth noting that every one of her abilities have a CC/utility component to it, in addition to doing a substantial amount of damage, this makes her a very good counter to most types of champions,

pros:
she does damage...
she is very versatile and able to get away from most situations
cons
she dies easily

I usually mid with her, and while champion kills will buy you most of your late game items, always make sure to last hit minions for as long as you can. as an assassin, you are not meant to be in the middle of a team fight. eliminate their main dps, then avoid taking damage for the rest of the fight. her chain is a good way to shut down a melee champion for a good portion of the fight. staying in the bushes is a good way to make sure you can kill without taking damage.