The Application output in Xamarin Studio clearly states:
The app delegate must implement the window property if it wants to use a main storyboard file.
Hence to fix : Go to your AppDelegate class and override the Window property as:
public partial class AppDelegate : UIApplicationDelegate
{
public override UIWindow Window
{
get;
set;
}
And that’s it!
See in action:
Nitin Pakhide BLOG
black screen Xamarin.iOS AppDelegate