Hello! I'm Zeymad and this is my Blitzcrank guide.
I will go over various builds their pros and cons and and how in my opinion Blitzcrank should be played.

Personal note: Blitzcrank is one of my first champs. This guide is a compilation of months of experience + the the good tricks and thoughts of other Blitzcrank players.

+You will provide lulz.
+You will instill fear in your enemies.
+You will initiate like no other.
+You are strong 1vs1.
+Your KDA can be 0/6/16 and people will love you for it.

-Your KDA can be 23/11/17 but you can not carry- your damage dealing teammates need to be good.
-You need to be good with your skillshot.

*Marks of Desolation since you will be doing physical damage mostly.
*Seal of Clarity since Blitz is mana hungry and his survival depends a lot on mana remaining.
*Glyphs of Focus/Warding(cdr,mgresist) Focus if you want to cap cdr without blue golem buff. Means you will have almost constant power fist (knockups and double AD damage every 2seconds) You will also need potion of Brilliance for this.
Warding glyphs are good against high burst casters. In an average long lasting game for me I have around 3000hp.. my passive will trigger @ 20% of my hp that means 600hp. In endgame however there will be AP casters or even AD carries who can crit or just burst 800-1000damage and negate my passive and I will die making all the mana I have for my passive worthless. So basically Mresist is so u have enough survival to get your passive going. Also this build will not have much magic resist so it will be definetly useful.
Glyph of Alacrity (AS) is attack speed in synergy with my W and trinity to go after carries and apply slows more successfully. Increase dps and on hit effects.
*Quints of Swiftness - more speed = easier to gank.. land hits and get fewer hits when in a fight. I figure more speed is more useful in both offence and defence (avoiding skillshots etc)

0/9/21

9 points into defence for extra armor/mresist +hp/5sec which increases to 30hp/5sec by endgame.

21 in utility for the respective summoner spells, cdr, mana regen, speed and fast leveling.

It is all focused on survivability and lane presence and to boost what Blitz is best at.. ganking.

I start out with mana gem because it equals 1 grab 2 power fists or if ur passive is triggered 100 extra hp. Later it should build into tear of goddess or catalyst because you want to get the maximum benefit from these 2 relatively cheap items in earlier game. I'd recommend getting Catalyst first.

I pick ward as my second item because Blitz is an excellent ganker that coupled with teleport summoner spells can reak havoc. Also map awareness is crucial to winning games. So why not start at level 1?

Mercury Threads.. should be the third item you complete. It gives you magic resist and tenacity against annoying crowd control. I used to play around with boots of swiftness or ninja tabi but Mercury Threads are just brilliant.

Next up is armor or magic resist.. look at your enemy team.. who is(are) the main damage dealer(s)?

Now your main tankiness is complete.. Blitz has relatively high damage output @ lower levels but this damage ain't as strong in lategame.. that's why after a solid midgame u'd want to start building items like phage/zeal or sheen. I prefer building phage for the extra hp and on hit slow effect. Kiting carries such as vayne can be quite a pain since a skillful player can rather easily dodge a grab.

Level 1 - Rocket Grab most of the time but it has a high cooldown 20sec (before cdr)
and it costs 140mana also it's 60 magic damage.
On the other hand if you picked power fist @ level 1 and if u can suprise your enemy then that
would be around 50 AD doubled into 100 physical damage (before armor resist) 25mana cost and 9 second cooldown so if there really is a level 1 fight. Power fist will give you more survival, more damage.
It's situational.

Other then that you should max your R > E > W > Q with an occasional flavor depending on the opponents you face.

Teleport - getting faster into teamfights via wards or minions. Pushing that lane where a hundred enemy minions are about to destroy your tower. You are an initiator and you need to be where the action is.
Ghost - Mostly due the ability to ignore unit collision. Run through the enemy team to disable that support/carry. Increased mobility too. Both offensive and defensive.

Unfit for jungling as it is now. Vamp doesn't work with powerfist as it should.
No natural hp restore.

Bad farmer/pusher til level 6. You can consider buying 5g/10sec items.

Blitz is not a real tank. He lacks natural armor, taunt. You can't be in the frontlines and draw enemy fire.You will die to fast and be worthless since your power lies in being able to cc the battlefield. You are best as an AD offtank protecting your carries on the side while pulling away enemy champs from your carries, as an anti carry or when filling a unique role as an AP burst type.

In higher level of play Blitzcrank will need a main tank such as Rammus to be the meatwagon.

Always tell your teammates what your planning. It's not just about landing a grab.. it's about landing a good grab... not grabbing amumu into your team and having him ulti.