This is a demonstration of writing a custom renderer for a blurred image for Xamarin.Forms on iOS and Android. I didn't implement Windows Phone because I don't know how, but pull requests are welcome!
iOS uses a gaussian blur from CoreImage. Since this can take a bit of time I do the blur on a background thread. I was able to just create a subclass of UIImageView and override the Image property setter to do this automatically, and therefore I was able to subclass and reuse most of the base ImageRenderer class.
The core of the Android blurring implementation came from a Xamarin recipe. The rest of the renderer is basically a copy/paste of the stock ImageRenderer because unfortunately the default image renderer is not designed well for subclassing. It uses a custom, internal ImageView subclass, which makes it difficult to use the same trick I used on iOS. It also doesn't provide any hooks for replacing the image in-flight. Therefore I made a whole new renderer that just did everything that the base renderer does. It relies on a bit of reflection to handle the setting of the IsLoading property on the Image element, which sucks, but that's the best I could come up with.
Obviously this is brittle, and I'd like to see the base renderer become more extensible in the future.