First of all: Most of the skins are not canon.
However, in the case of that skin, it was canon before the Retcon. Even if he is from Zaun, Zaun and Piltover are connected and Blitzcrank is not an enemy of the People of Piltover. In the old lore, the Piltover Custom skins were created by Heimerdinger and Corki... and yes, Blitz went there for a cool paint job
https://www.mobafire.com/images/champion/skins/landscape/heimerdinger-piltover-customs.jpg
http://www.leaguesplash.com/wp-content/uploads/2013/09/GattlingCorki.jpg
http://leagueoflegends.wikia.com/wiki/The_Journal_of_Justice:_Volume_1,_Issue_04
Flames for Your Wagon
The city-state of Piltover is the leading center of ecologically-minded techmaturgical and magical research in Valoran. If you are looking for the latest crazy gadget to give your ride that extra get-up-and-go (that won’t shatter the planet into a billion bits in the process), then look no further than Piltover Customs. The proprietor and chief mechanic of the most successful transportation customization and modification enterprise in the City of Progress is none other than Corki, the Daring Bombardier.
"In between my time in the League and my duties with the Screaming Yipsnakes, I like to mess around under the hood of all sorts of vehicles!", Corki explained as he gave a special tour of the workshop he and his crew of technicians use to make some of the most radical conversions you’ve ever seen.
"Most of the mods we do are for caravan wagons – adding armor plates or creating slits for archers inside the wagon. Every once in a while, we’ll get to work on an order for some really over-the-top stuff – state of the art hextech. That’s where the real fun is!"
For Corki’s tougher assignments, he will often turn to his comrade-in-arms Heimerdinger, the Revered Inventor. A number of their more interesting contraptions are on permanent display in the Yordle Academy of Science & Progress, which is located in Piltover. A trip to the City of Progress is not worth talking about unless you spend some time checking out the latest gizmos that Corki and company have put together. The Yordle Academy of Science & Progress will grant tours nearly every day of the week, while Piltover Customs will provide tours by special request only.