Blitzcrank can be built as many roles. Bot Tank, support, ap-burst, ad damage etc. The role I usually go with is kind of hybrid of all roles in a team. Blitzcrank do not specialize as a specific role, then why build him as a specific role?

He functions well as a tank because of his passive - Mana Barrier, his good armorscaling (3,5 per level), hp- (100 per level) and magicresistance-scaling (1,2 per level). Stacking mana will surprise the enemy team when you suddenly have full health again - which gives you teammates hopefully enough time to kill the majority of your opponents. The passive is your lifesaver, and watch your mana-bar close.

This guide is not based for ranked play, since I haven't tried it out in ranked matches much, because it usually is no room for Blitzcrank in ranked matches unless your team builds around him.

Why Clarity
Because it functions just like heal for blitzcrank. It also allows you to stay in lane for much longer, and your teammater will benefit from it because you're giving them mana as well. Laning with a mana-user is not neccessary, but preferable for the team. You already have an escapemechanism, Overdrive.

Why Exhaust
Because that way you and your lanepartner can score some early kills by pulling into the turret, Power Fist and Exhaust. The spell also gives you survivability because you can use it against chasers when you're at low health.

Other options could be Flash and Clarity or Flash and Exhaust. Because Flash is an insane spell that can save your champ's life every time you use it if your teammates and yourself is good enough. I don't usually pick Flash, but I know I should since it's very easy to lose control of Blitzcrank during teamfights because of his fast movementspeed.

The runes I recommend is Greater Mark of Desolation, Greater Seal of Armor, Greater Glyph of Cooldown Reduction and Greater Quintessence of Health, Greater Quintessence of Desolation or Greater Quintessence of Movement Speed.

Greater Quintessence of Health - If you're into early-game harassment, and scoring some early kills.
Greater Quintessence of Desolation - Same as the above, but you're a more experienced player who can rely on your expert-positioning and pro skillshots.
Greater Quintessence of Movement Speed - If you want to focus on mid-game and ganking, which I think is the funniest playstyle of Blitzcrank.

If you want to you can go with Greater Seal of Evasion combined with 9/12/9 masteries, to give you more sustainability and tanky style of play.

He's a little bit of everything

Since my opinion is that Blitzcrank can be played as many different roles, the masteries I mean is the most suitable for him is 9/7/14, 9/12/9 or 9/9/12.

9/7/14

Why so varied talent-tree?
Because Blitzcrank can be played as many different roles in a team - he can tank, he can carry both as AP or AD and he can support or crowd-control.

Offense
1xCripple and 3xArchmage's Savvy
4xSorcery
and one point in Archaic Knowledge

Defense
3xResistance, 1xHardiness
3xStrength of Spirit <-- If you're not level 30 I recommend that you should have this talent - since it will have the most impact on the game. Lategame it can give you over 30+ hp5, and even more combined with increased mana, and healthregeneration from the utility-tree.

Utility
1xGood Hands, 3xPerservance
4xAwareness OR Expanded Mind <-- Whatever you prefer really.
3xMeditation and 1xInsight
2xQuickness

9/12/9

Offense
Cripple, 3x Archmage's Savvy
4x Sorcery and
Archaic Knowledge
For mid-lategame magic-penetration and cooldownreduction to give u 8,89 total CDR earlygame.

Defense
3x Resistance , 1x Hardiness , 3x Strength of Spirit , 4x Evasion and 1x Nimbleness
This setup gives u lane-sustainability in form of good hpregen ( Strength of Spirit + Manamune is a good combo)

Utility
3x Good Hands , 1x Perservance
4x Awareness
1x Insight
If u die, u respawn faster (You could also go with Perservance if you want to, it gives you just a little more regeneration, but may be worth it.) It is better to reach maxlevel faster. And of course Insight will give your teammates some extra mana.

9/9/12

Offense - same as above

Defense - Same as above without 4x Evasion and the 1 point in Nimbleness

Utility
3x Good Hands , 1x Perservance
4x Awareness
3x Meditation and 1x Insight

This gives u even more lane-sustainability and better chance to score some kills.

The reason to max out Overdrive first is because it gives you can easier lasthit minions with it activated, and you can get from lane to lane much faster and therefore allow your team more ganks and hopefully more kills. I think Rocket Grab should be used early only of it's pull, not for dealing damage. Overdrive and Power Fist will deal more damage and will help you stay alive.

So Overdrive first for movementspeed and attackspeed early, then Power Fist for crowd-control during mid and lategame, then Rocket Grab for damage, and not letting enemies get away, during the lategame.

This playstyle uses Blitzcrank as an AD-champion early, then focuses more on AP, CC, and magicdamage later during the match.

Always start with a Sapphire Crystal.
It gives you more mana, and more hp-regen because of our talent-tree, and more hp because of the shield. After the crystal you can go with whatever that is neccessary, Sheen for kills, Manamune for lots of mana, Catalyst the Protector for building Rod of Ages or Glacial Shroud for low cooldowns and heavy CC. If u take Manamune first, Clarity will not have the same effect - but will still be useful for your team.

Firstblood can be quite easy when you're playing as Blitzcrank. Most people in normal-games don't really have it in mind that your shield will save you, and they start chasing (and sometimes diving) you to the turret. After the turret have dealt some hits, and the opponent is trying to save himself, you just pull him back in. If you're going for firstblood in your lane (bottom or top), hide in the first bush at the edge close to the next bush. Hide there because if they're trying to gank you, you will punch them in the face, and pull one of them back when they try to run away. Champs with low-cooldown stuns or slows (like Gankplank's passive or Twisted Fate's Gold Card) will be super combos with Blitzcrank

Earlygame you want to rely on attackdamage primarily with your Power First and Overdrive. That's why we have armorpenetrationrunes. Sheen+ Powerfist is a good combo.

Midgame I tend to rund around with Boots of Swiftness and Sheen or Manamune from lane to lane and pull for kills. Or you can also stay in lane and farm up for lategame.

With Blitzcrank you can choose 6 items, and build them in whatever order way you want. according to how your game is going.

In the lategame you should always try to initiate, you pull is devastating if you catch some of their squishies. Also if you run in first, the team will probably focus you right away, and when your hp go down, you shield will rescue you. Then you just run out of the teamfight, to have them switch target - and run back in again when you feel save enough.

This may not be the standard-way of playing Blitzcrank, but it sure is a lot of fun when you learn to use his Rocket Grab properly.

Please rate the guide and leave comments about what you like, dislike etc. so I can make changes for the better. :)
Thanks.