Blear

Transform your photos into stunning blurry wallpapers. Make your Home/Lock screen shine!
Drag the image horizontally to change the crop.
You get the best result with bright colorful photos, like a handful of jellybeans or unicorns.
The app comes bundled with some beautiful photos to get you started. Just shake to get another one.
When you have swiped your preferred blurriness, tap the save button on the right. In the Photos app, go to the image you just saved, tap the action button, and choose “Use as Wallpaper”.
The app also comes with an Action Extension. So instead of opening the app, you can start by finding a photo in the Photos app, tap the action button, and then choose “Make Blurry Wallpaper”.

Blear is open source: https://github.com/sindresorhus/blear