Hello and welcome to my own personal Blitzcrank guide.
What you can expect to find is:
-Runes, explenation
-Masteries, explenation
-Items, explenation
-Skill Sequence, explenation
-Summoner Spells, explenation
-Pros/Cons
-Early Game
-Mid Game
-Late Game

Okay, so as you can see above. I am totally just going all out for CDR, why?

Well, if you ask that question, then you've probably not played Blitzcrank YET, but I am here to help.

The reason as to all that CDR is because of the fact that; The quicker you can use your abilities, the more totally awesome damage you can dish out in a low amount of time.

Rocket Grap, Overdrive, Power Fist and especially that lovely Static Field, they all contribute to do some awesome damage, aswell to just getting out of a nasty situation that you wish you hadn't been in.

With that, lets move on.

Okay, so the masteries I've gone for is primarily Utility, and then branching into Offense for those extra 3% CDR. Those I've taken in Utility seem fairly self explanatory, don't they?

Extra CDR, CDR on summoner abilities, increases movement speed, lil' bit more GP10S (Gold per 10 secs), mana and Mana/health Regen.

Blitzcrank's Passive, aswell as Manamune, both work very well with the extra mana. Meaning more damage absorbed, and more attack damage.

With that, on to items!

So as you can see, I've obviously gone for being able to move quickly about, armor, CDR, Attack Damage, mana/HP, aswell as a FAIR amount of crit chance. (+ a bonus low amount of AP for extra damage on Static Field)

(By the way, you'll have A LOT more than 168 attack damage at level 18, due to the special effects of Manamune/Atma's Impaler.)

All of the above named stats are some that are AWESOME to have for a damage Blitzcrank. And the large amount of Mana/Mana Regen will mean that if you get low health, you'll suddenly have an additional (about) 1850 health due to Mana Barrier (at level 18), which can really give the enemy that "Ohh ****" moment and suddenly give you the upper hand.

At the end of a match, you'll probably be extremely beefy, aswell as hit pretty darn hard and you'll be a certain B to take down 1v1, or even 2v1 due to the amount of zerg, armor, health and immense running speed you have.

With that finished, NEXT CHAPTER!

Now, some might wonder "Why take Rocket grab first instead of Powerfist?"

The explenation is fairly simple. Even though Rocket Grab takes a majorly amount MORE than Powerfist, Rocket Grab can
give you ridiculous big advantage over someone who's fleeing due to the fact that you can just grab them into your range again, stun them for 1 sec with them, and have time to do atleast one auto hit.

Powerfist as the second due to that it's a great follow up ability to do AFTER Rocket Grab. Just pull them in and then knock them up into the air.

Third is of course our lovely Overdrive. Well, isn't it self explainatory? It's a nice substitute for you until you can get your boots, and allows you to escape a gank with greater ease.

The reason I don't upgrade Powerfist once more, before I am "Forced" to do it, is because that after rank 1 of the ability, it doesn't get any stronger or better. The cooldown is just reduced, where as the two others benefit more from being skilled up quicker.

So, why did I put in Clarity aswell as Ghost. Especially that ghost when you already have Overdrive?

It's simple. When Overdrive and Ghost are up at the same time, there will only be ONE champion in the whole game able to outrun you, and that's Rammus, whom you should be able to solo in most occations due to your Beefyness whilst still dealing huge damage. (In some occations, Yi can aswell if he goes for high Movement speed, too. But in most fights I've been in, he's quite squishy)

Clarity also does explain itself. You want to be on high amounts of CURRENT mana, the most of the time you ever possibly can, Clarity is brilliant for that.

On to Pros! (And cons)

Pros:
- Extremely beefy late game
- High Zerg late game
- Extremely awesome for team fights if using Rocket Grab/Powerfist combo.
- Can outrun mostly anyone.
- Is able (and good at) doing "Hit'n'Run" for harassment after level 10-ish.
- Can completely ruin a gank with his Mana Barrier.
Cons:
- Is kinda weak until Manamune (Or until level 6, whichever comes first) is achieved, due to not hitting all that hard with either Melee/spells.
- Is sort of slow until you get your first few levels.
- All in all not that strong EARLY into the early game.

Okay, so what you want to do ask quickly as possible is to pick up a Sapphire Crystal and a mana potion quickly and then run out into one of the two side lanes together with someone else whom you can have a good chance to lane together with (The best for your to lane with are usually spell casters, because they can do some fairly good damage even early on)
Whilst running to the lane, pick up Rocket Grab.
While lanning, try to as often as possible to last hit a minion, and remember, try to not waste Rocket Grab on anything but harassing the enemy champions, force them to go back, or something else. Don't waste it on the minions, due to it's pretty big mana cost and cooldown.

By now, the enemy will know you have Rocket Grab, and will probably "fear" getting close due to the thing you can pull them in for ganking. Just go around doing this until you can afford Tear of the goddess, then recall and get it. At this point, you'll probably be about level 3, or half way through it. Unless you're good at getting the last hit at which you might even be level 2 and already have it. How ever it happens, good. Just remember the skill sequence.

Go out again and use Rocket Grab to harass them even more now, due to the fact you have even MORE mana now, and will only get more each time you cast a spell, aswell as some mana regen.
Try to hang around your turret, because if you can get to it, then you SHOULD pull in an enemy champion using Rocket Grab and then knock them up into the air while they're still close using Power Fist. Which will usually either kill them, or leave them with a very low health (Which at this point will be easy to chase down and finish). Keep going around until you have enough gold for a Manamune. Then recall and get it, ta-da, you suddenly have like 110 attack damage or more.

With the newest patching, then you have two choices. Keep lanning until you can get either the Ionian boots of ludicity, or the boots of swiftness. If you have no trouble chasing, then get the Ionian boots for even MORE CDR (love that stuff). Welcome to Midgame.

Right about now, you should have either Ionian boots/Berserker Greaves and a Manamune. Aswell as probably be level 6, or close to 6. What you want to do now is actually start being one of THE biggest harassers in the game due to the fact that you can pull people in, seem like you're completely over extending due to having "Low Health", and then have Mana Barrier aswell as being able to run away to just save your rear 24/7. If you are to zerg someone, get ready to do it like this

Powerfist->Overdrive->Rocket Grab->1 passive tick from Static Field->Active Static field. At this time, doing it like this should PROBABLY award you with being able to do some solid 800-900 damage VERY quick zerg, which will probably make them retreat or even run away to recall. If you're lanning together with a spell caster, they'll probably be dead (Unless it's Tryndamere.. Whom'll probably twirl away).

Just keep doing this in order to farm gold as quickly as possible. When you can, then take out the dragon too for some solid whole team gold/xp. Aim to get Atma's Impaler and Glacial Shroud for some even more damage, aswell as some solid armor/CDR. At this point you'll be about.. Hmm.. Level 9-11. And ready for late game.

Okay, so lets summarise:
At this point you'll probably level.. 11-ish, you should have Manamune, Boots of Swiftness, Atma's Impaler, and Glacial Shroud, if not.. Well your team is probably getting juked then, aswell as you.

If you do, hurray! You're well on your way.

What you wanna do right now is farm up your gold so that you can Frozen Heart. It works very well with this build giving you in total an additional 5% to Glacial Shroud, slows everyone around you, making it easier for you to chase/run away. Gives a solid amount of mana, and a beefy 99 armor.

At this point is probably where the big team fights break out, or you'll be going around in gank groups to get the enemy out of the area. For both purposes YOU'RE THE RIGHT GUY! Either you can just pull them in and be an overall pain in the *** just by that, in addition you can knock them up into the air at which point they're probably dead if you're in a gank group. BUT in the case they're running away, you can use Rocket Grab to pull them back in if they're not careful enough. Just keep following my item and skill sequence in the guide, and you should do good. Remember, you wanna be in those team fights as the crucial part that can make or break a gank. Remember, you've a big robot who can go around quoting the Dalek's by saying "Exterminate. Exterminate.", how much more awesome can you get? (jokes aside).

If nothing is currently happening, try to go around and backdoor their towers, at this point you should not have a hard time soloing down a tower with your amounr of damage and the attack speed bonus Overdrive gives you. Just remember, Powerfist CAN be used on towers for double damage.

And with that, I will conclude my guide. Constructive criticism appreciated, trolls can just stay away.