Это должно помочь вам. Просто позвоните FadeOut(myButton)
:
private void FadeOut(UIElement fe, int seconds = 2)
{
DoubleAnimation animation = new DoubleAnimation() { To = 0, Duration = new Duration(new TimeSpan(0, 0, seconds)) };
Storyboard sb = new Storyboard();
sb.Children.Add(animation);
Storyboard.SetTarget(animation, fe);
Storyboard.SetTargetProperty(animation, new PropertyPath(UIElement.OpacityProperty));
sb.Begin();
}