I'm seeing a bug when setting Image.Opacity to any value between 0.0 and 1.0. It will blend the image (or the background color if there is no source image) with black, instead of becoming translucent and showing the elements behind it. This happens when you set Opacity in XAML or code, or via the FadeTo animation.
Note that once the Opacity actually hits zero (0.0), the image correctly disappears and the elements behind it become visible.
I'm using Xamarin.Forms 1.4.4.6449, tested only on Android so far.
UPDATE: Looks like this does not repro on an Amazon Fire HD (3rd gen). Opacity blending works fine. So far it only repros on the Visual Studio Emulator for Android (tried KitKat 4.4/19 and Lollipop 5.1.1/22).
This actually affects other element types besides Image. A simple ContentView with BackgroundColor set to "Red" and Opacity to something less than 1.0 will blend with black until it gets to zero.