Hey,
I am CanuXis, I am just a silver player using this site as a way to take notes. Hopefully I help even just one person along the way. Any notes on improvement or information you may find helpful is appreciated.

So what is Blitzcrank's identity?

While there is no one phrase or word to describe it we can break it down.

CD dependant
Immobile
Likes Picks
Likes Fast Fights
Not Much depth(?)
Power budget in Q
Tank
Vision Demon
Weak against mobility
Works well with burst

These will help us get the baseline for what Blitz PREFERS as a character. Of course Blitz can work outside of these conditions but if you plan to have a pool of champions it is good to understand when to pick him.

I will go more in-depth at some point but not as of right now.

Keystone

Aftershock: Immobilizing an enemy champion grants you 35 bonus armor and bonus magic resistance for 2.5 seconds. After the duration, you release a shockwave that deals 25 − 120 (based on level) (+ 8% of your bonus health) magic damage to enemy champions and monsters within a 350 radius.

Aftershock is used for two reasons. The first is for the extra burst. This can help with extra damage your team may lack. Like if you have an Ashe. The second use of it is to give Blitzcrank a damage mitigation tool. The only one in his kit being his passive Mana Barrier which is on a 90 sec CD. This allows Blitz to tank a lot more than he would otherwise. It also gives health scaling which more easily allows for him to build items such as Warmog's Armor.

Tier 1 Rune

Demolish : Your next basic attack against a turret with 6 stacks is empowered to consume all stacks to deal 100 (+ 35% of your maximum health) bonus physical damage.

Health scaling allows it to scale with Aftershock. Helpful for snowballing lanes.

Font of Life: Slowing or immobilizing an enemy champion heals you and the nearest and most wounded allied champion for (Melee 10 − 50 )(based on your level).

Extra sustain in lane , used for nuetral or losing lanes where plates are scarce.

Tier 2 Rune
All of these are viable but some are more niche than more general ones.

Conditioning: After 12 minutes, gain 8 bonus armor and 8 bonus magic resistance and your total armor and total magic resistance are increased by 3%.

Adds a bit of tankiness.

Second Wind: After taking damage from an enemy Champion icon champion, Health regeneration icon regenerate 3 (+ 4% of your missing health) health over 10 seconds.

You want to use Second Wind in lanes with a lot of poke. Often though you can mitigate poke lanes by either pushing them in, requiring thier abilities be used for farm, or by not being in the lane because you are support and don't need no adc. In reality it's only real use is in a game with little to no roaming oppurtunities.

Bone Plating: After taking damage from an enemy champion, activate Bone Plating for 1.5 seconds. While active, the next 3 spells or attacks you receive from the enemy champion that triggered this effect deal 30 − 60 (based on level) less damage.

Use against burst heavy or all in lanes. Also useful against other Hookers. Mainly used to mitigate early before you get tank stats.

Tier 3 Rune

Overgrowth: Every 8 monsters or enemy minions that die near you permanently generates a stack. Each stack grants 3 bonus health.This effect can stack indefinitely.After reaching 15 stacks (120 monsters or minions), your base and bonus health are permanently increased by 3.5%.

Overgrowth gives late game scaling and gives more health to compliment tanky stats. It also allows for comfortable building of Frozen Heart and since you still accrue health, benefitting more from the armor you're given.

Unflinching: While you are slowed or immobilized, gain 2 − 10 (based on level) bonus armor and magic resistance, lingering for 2 seconds after the impairment ends.

Use against comps with lots of CC.

Secondary Runes

Inspiration

Hextech Flashtraption: Charge up a flash for up to 2.5 seconds ( max distance at 1 second) when Flash is on cooldown.

Good for getting unique angles to hook.

Timewarp Tonic: Upon consuming a health potion you heal yourself equal to 40% of the total health it would restore.

Good sustain. Alternative to biscuits if you are not worried about mana issues. Good when paired with Frozen Heart rushes or Soul boots that pretty much mitigate any mana issues.

Biscuit Delivery: Receive a Total Biscuit of Everlasting Will at 2:00, 4:00, and 6:00. If the inventory is full while receiving a biscuit, the biscuit(s) will appear as soon as a slot in the inventory is available.

Sustain for Health AND Mana. Try to use when lower as it recovers health and mana based on how much you're missing.

Sorcery

Nimbus Cloak: Using a summoner spell grants Ghost ghosting and 5% − 25% (based on summoner spell cooldown) bonus movement speed for 2 seconds.

Gives Movespeed which is Blitz's only hope for closing distance besides fishing hooks which is inconsistent. It also gives more burst movespeed to ctach people with E for guarenteed Hooks.

Celerity: All movement speed bonuses increase in effectiveness by 7%. Additionally, you gain 1% bonus movement speed.

Must pick with Nimbus Cloak which gives an even greater boost to speed and boosts the MS stim from Blitz's Overdrive.

Keystone

Glacial Augment : Immobilizing an enemy champion will cause 3 glacial rays to emanate from them towards you and other nearby enemy champions, creating icy zones with a 700 unit radius that last for 3 (+ 100% of the immobilizing effect's duration) seconds. Enemies in the rays deal 15% less damage to allies.

Makes it harder for enemies to escape and gives huge damage reduction to allies. Also allows you to hold E for when the slow ends extending the impairment duration (I reccomend to do it against flashless, immobile targets otherwise the target can just use mobility or flash to escape).

Tier 1 Rune

Hextech Flashtraption: Charge up a Flash for up to 2.5 seconds ( max distance at 1 second) when Flash is on cooldown.

Good for getting unique angles to hook. Really good combo with nimbus and Celerity allowing for really creative uses to catch people. The other two options don't do anything for Blitzcrank.

Tier 2 Rune

Biscuit Delivery: Receive a Total Biscuit of Everlasting Will at 2:00, 4:00, and 6:00. If the inventory is full while receiving a biscuit, the biscuit(s) will appear as soon as a slot in the inventory is available.

Sustain for Health AND Mana. Try to use when lower as it recovers health and mana based on how much you're missing.

Tier 3 Rune

Cosmic Insight: Gain 18 summoner spell haste and 10 item haste:

Huge for Blitz. Has good combos with Celerity giving more MS per Summoner Spell, items such as Locket of the Iron Solari have more uptime, and gives lower CDs to support items.

Secondary Runes

Resolve

Conditioning: After taking damage from an enemy champion, activate Bone Plating for 1.5 seconds. While active, the next 3 spells or attacks you receive from the enemy champion that triggered this effect deal 30 − 60 (based on level) less damage.


Unflinching: While you are slowed or immobilized, gain 2 − 10 (based on level) bonus armor and magic resistance, lingering for 2 seconds after the impairment ends.

While you can swap these out, the Glacial keystone build is less about being tanky and as such has a lot less damage mitigation to compliment the health you can get from Overgrowth making resistances and mitigation more important than health. Play how you choose though.

Sorcery

Nimbus Cloak: Using a summoner spell grants Ghost ghosting and 5% − 25% (based on summoner spell cooldown) bonus movement speed for 2 seconds.

Gives Movespeed which is Blitz's only hope for closing distance It also gives more burst movespeed to ctach people with E for guarenteed Hooks.

Celerity: All movement speed bonuses increase in effectiveness by 7%. Additionally, you gain 1% bonus movement speed.

Must pick with Nimbus Cloak which gives an even greater boost to speed and boosts the MS stim from Blitz's Overdrive.

With Cosmic Insight the Sorcery tree gets way better and Blitz gets a lot more roaming opportunities coming in like a budget Hecarim with how fast you can run.


Make sure you understand how Blitzcrank works with these runes pages.

With Aftershock Blitz plays more towards a more burst oriented frontline playstyle utilizing the damage mitigation and health sacling to minimize his own lack of built in reduction.

With Glacial Augment Blitz plays as a Utility champ, sacrificing tankiness in favor for peel and more frequent summ usage. It also his heavily focused on a single roation as the lack of tanky stats make it so Blitz does not often have two rotations of abilities in a fight.

I am new to making this stuff so the formatting is difficult for sure (OMG I suck at coding)! I will add more and change stuff up but for now . . .

Thank You!