This is how I build Blitzcrank and I have successfully helped my team win countless times thanks to that master initiate and the ability to save team mates. Blitzcrank is also one of the best counter against casters, especially in teamfights because of his awesome ultimate.

Don't rate it if you haven't tried it please.

Pros:
Master of initiating
Very good cc
Relativly high damage
Awesome ultimate
Very good at escaping

Cons:
Slow early game
Low base mana
High mana consume early game

For Blitzcrank I use very simple runes which seem to fit him perfectly. I have Magic Penetration Marks, Health/Level and Mana Reg/5 Sec Seals. For Glyphs I use Cooldown Reduction/Level so I can make my spells more spammable. At level 18 the ultimate has a cooldown of only 18 seconds. The Seals give me more survivability and just a tad more sustainability in lane. The Marks and Quints just gives me more damage. This setup has worked out very smoothly and I have had no problems with it yet.

The Masteries are all about making me more durable and efficient in lane, that's about it.

The items I picked works very well since they make me very hard to kill aswell as doing a lot of damage. The cooldownreduction at level 18 will be capped and that is going to highthen the DPS and make it alot easier to farm and win teamfights. Of of the points of this build is also to maximize the Passive ability Mana Barrier. The more mana Blitz has, the better it gets.

The Skill Sequence is pretty basic. Since you are not out to get kills you should do this W>Q>E>R. By now the person you grabbed should be dead with a little help from your team mates. You are not one to go SOLO vs others except squishies. Make sure your team is always around so that YOU can help THEM get kills and help them not to GET killed. Be a master of initiation AND disruption. Be a pain in the *** and be proud of it.

The summoner spells are pretty basic aswell. The Flash is for chasing and/or escaping and the clarity is for helping you and your laning buddy to stay in lane for longer. Farming is key.

This build rips, mark my words you will not be disappointed. BUT don't believe that you can use this build and get good with Blitzcrank instantly. It takes skill to land his Q at the right times and playing smart is what seperates the good Blitz' from the great. Don't forget to have fun playing him :)