FYI first guide, I dnt read up on the forums or anything, this is just my opinion.
This Blitz is meant to have the ability to do dmg, have a strong mana shield, and be able to get away when needed. (This Blitz is a good initiator, but not great at tanking for more than a moment vs multiple enemies.) again IMO/with this build

To use this to it's potential as an initiator u must know when to hold em, kite, and run.
Hold em- If You've initiated and can help eliminate the enemies without them killing you, by all means, do it.

Kite- If You've initiated but are pretty sure you'll die before its all over, start pulling the enemies, making them chase and allowing ur allies to deal damage while u can avoid taking most.

Run- if You've initiated into massive burst without realizing or just get in a bad situation, pop cleanse(if needed) and book it with w or ghost/flash.

I chose Cleanse/Ghost but flash is easily exchangeable with ghost and serves almost the same purpose.

The reason for this is these spells allow you to easily retreat once you've initiated a teamfight.(of any size with multiple allies/enemies)Allowing your survival and having been focused you're team will likely have the upper hand.

Using the Cleanse/ghost strat to escape is more effective at allowing the enemies to start to chase compared to cleanse/flash. (so u can kite) Where as Cleanse/Flash would allow for a safer escape. Cleanse/Flash being my first choice since you already can use his W for speed.

Using Clairvoyance in place of one of these spells allows you to be a better help to your team. Also, your can use it on brush, around corners, over ridges to allow for an easy grab.

I chose These runes because they're useful for a range off offensive AP builds. They weren't meant specifically for Blitz.

If the masteries/skills aren't self-explanitory...
His masteries were meant for AP and Mana/Reg for his sheild. (When it pops, great sign to get out)

As for skills, Q does what it needs to with one point, W and E are good to level simultaneously, as for R, OF COURSE, get that up when u can!

Might notice they're all Recommended items or part of a recommended item.
Thats because they work. Only thing to add is in massive AD or AP, get Frozen Heart or Banshees veil first, respectively. Also, Getting Meki Pendant as a start item works better IMO if you have no mana reg runes.

Pros: Great Initiator,
Great at escaping,
Great at Getting free kills,
good damage for an off-tank.

Cons: Could have more survive ability,
Could have more damage,
not the easiest champ to play.

Heres some I can Think of...

Use W when at your spawn. Your mana will regen before you get back to the fight, so use that speed boost.

Using E to help last hit more is quite acceptable, only a few seconds cooldown once u get cdr from frozen. Also, after Tear or Manamune you will have completely regened the mana used after 4-5 seconds.

His Ult has a 30 second cooldown, relatively VERY short, dnt be afraid to use it to farm if u dnt see a fight in the near future.

Buy vision wards when u have extra money after buying an item. Not only when used effectively can these seriously help win a game for your team. But, as Blitz, many times, teams end up in a stand off near brush, or around a corner, or over a wall/ridge, and a vision ward can allow you to see where the enemies are to lay down a grab for a free kill.

Lane with high DPS champs, Using Q/E to CC an enemy leading in a kill for your lane partener helps to feed them giving your team a good advantage.
And as with any champ, know your enemy and watch the map. Knowing your enemies capability of downing you and how long they take to do it. This allows you to initiate and escape without letting the enemy champion(s) get a kill from focusing an off-tank.

The Cleanse/Ghost or Flash allows you to be able to fight more and be a little cocky. Use this to have the enemy focus you and then escape hopefully killing or at least getting an assist on an enemy or two.

If you don't have any very mana dependent allies, Blue buff can help you a lot with it's regen (allowing you to keep you manashield high becuase you WILL need it now and then) and CDR. (Rocket grab has quite the cooldown, less cd more free kills)

A Blitzkrieging Blitzcrank makes a great off-tank that has survive ability and maintains a decent offense. general strat is a build to initiate, damage, and kite/flee making it out alive while holding focus and tipping odds of team fights in your favor.