Leblanc has to be one of my favorite champions even though I surely don't play her often enough. She has a quick burst and can open up many team fights in the correct direction. LeBlanc, even though she has a high burst, is actually better used when maximizing the utility of the spells in her kit. For instance you want to burst down the ADC right away but after that people assume she is useless.... Hence upping the chains for early ganks, and dominance in CC. If played right she can completely shut down three enemies in the team fight. Make sure you use the W - R combo only in dire need, seeing as W has the longest cooldown out of her spells.

Magic Pen. Marks for early damage against their mid lane.
Ability Power Per Level Seals for the damage increment over time, same with the Glyphs.
Quints can be a tad different from mine, sometimes I run the Movement Speed Quints while others a I decided to go FLAT AP.

Standard offence for mages, picking up 3 points into Brute Force for the last hitting.
Utility for m.regen as well as Summoners Insight and increased buff timers.

MAGIC PEN....MAGIC PEN.... MAGIC PEN. Cannot stress it enough on Leblanc. Seriously though Magic Pen. Take Rabadons for the nuke....and Zhonya's or Rylais for durability depending on the situation.

While most guides say to up W after Q... Ethereal Chains or E is way more beneficial than W, for teamfights... for soloing and for CC. Always take the ult points once you get 6, 11, 16...obviously.

Flash and Ingite... Pretty self explanatory but, Flash for quick gap closing or get-aways. Ignite for true damage over time and the Greivious Wounds effect.

Keep an open mind with Leblanc... she is frustrating at first but gets extremely beneficial once you figure her out. Also, don't be those people and rush for kills. TAKE THE TIME TO CS. She actually can CS well... She just can't push as strong as other champs. Do not let that collide into being a poor cs-er. Remember to also not over-poke the enemy, as easy as it may be to do so... Let them linger around at about 50%-55% hp, and then rush burst them. ESPECIALLY IF THEY ARE ALONE. Hope you found this guide helpful, if not then sorry to have wasted your time!