I like Blitz, He's a fun and funny champion. Many 'lolz' to be had!

Since this is a Support build I chose items which mostly benefited the team over blitz himself. Some of them also work well building from the money items.

Giving blitz the Shard of True Ice was more of a forced decision, being built from Kage's lucky pick; it just follows the flow better and saves money in the long run.

Boots of mobility are great! That's why I chose them!
Nah, being able to move quickly is key to hitting Rocket grabs(q) on difficult targets. combined with your Overdrive(w) you can dart in and snatch the AD Carry, or who ever, right from under their noses! Easy team fight after that!
Remember Blitz's main strength is being able to pull apart teams, if you don't hit a single grab, your team is likely to lose.

Frozen heart is an easy choice for Blitz. It's the main anti-AD item for you, and gives a massive cooldown reduction buff. All this and boosting your mana shield, you can't go wrong!

Shurelya's Reverie is a nice item through and through. the health, cooldown and regen attributes are desirable mid game, but it's speed boost is fantastic late game. combined with Overdrive(w), boots of mobility and captains enhancements mean your entire team can charge around the battlefield at a moments notice! Remember; mobility is a key factor in victory!

The sight stone can still be a controversial item for me. On one hand, I don't have to remember to buy wards, just place them. on the other hand, I can only have 3 out at once?
That's a difficult compromise. however, until late game, you can still buy extra wards should you need to. Still, the health is a benefit.

Runic Bulwark is a must for support champs. The pure defence gives your team a massive advantage late game. Also, being a melee champion, surviving the team fight and small skirmishes will keep you healthier later on, and the enemy not-so-healthy.

For the masteries I wanted to make Blitz a pain in the early game and get him gold asap.

Considering the nature of blitz's engagement, pickpocket looked like a good choice for creaming more gold out of each fight. But intending for him to get stuck in puts you in a risky spot. So to counteract the aggressive tactics for more gold, I've tried to put as much defence on top as I could. more health, more armour and a little more magic resist. hopefully that can give you a boost up to philosophers stone quickly and get you a huge lead on the enemy support.

Experimenting with gold quints at the moment, the look pretty snazzy for support, but might provide a conflict for survivability.

Other than that; health and magic resist for surviving. Attack damage because Power fist(e). and a little flat mana because mana shield at level 1 will be key to getting out of sticky situations and surviving early ganks before mid game.