Cannot resolve all property references in the propertypath - wpf

Question

I have a UserControl with a Border, the color of the border should be setted with a Dependency Property. I also want to animate the opacity of the border. My current xaml code looks like this:
<Border BorderBrush="{Binding ElementName=ImageViewerUserControl,
Path=NotificationColor}" BorderThickness="3" x:Name="AnimatedBorderBrush"
Visibility="{Binding ElementName=ImageViewerUserControl,
Path=ShowSequenceErrorNotification, Converter={StaticResource boolToVisibility}}">
<Border.Triggers>
<EventTrigger RoutedEvent="Border.Loaded">
<BeginStoryboard>
<Storyboard>
<DoubleAnimation Storyboard.TargetName="AnimatedBorderBrush"
Storyboard.TargetProperty="BorderBrush.Opacity"
RepeatBehavior="Forever"
AutoReverse="True"
From="1"
To="0.0"
Duration="0:0:1"/>
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</Border.Triggers>
</Border>
This only give the error:
Cannot resolve all property references in the property path 'BorderBrush.Opacity'. Verify that applicable objects support the properties.
But if I change to color of the BorderBrush to, lets say Black it works. How is this possible to achieve? I want to set the Brush color of my border via a dependency property. And yes, the dependency property is a Brush

Answer 1

I think the problem here is that the animation will only work if there is an object (the Brush) to animate. If you register your DependencyProperty without a default value it is null by default. Please try registering the DP with a default value
public static readonly DependencyProperty NotificationColorProperty = DependencyProperty.Register(
"NotificationColor",
typeof(Brush),
typeof(ImageViewerUserControl),
new PropertyMetadata(Brushes.Transparent)
);
Edit:
And as #Sheridan says use Storyboard.TargetProperty="Opacity" instead of Border.Opacity. Although it works if you specify a direct BorderBrush it doesn't worked for me with a bounded DP.

Answer 2

Your AnimatedBorderBrush name is misleading as it relates to a Border and not a BorderBrush. If you want to animate the Border.Opacity, then use Border.Opacity in the DoubleAnimation instead of BorderBrush.Opacity:
<DoubleAnimation Storyboard.TargetName="AnimatedBorderBrush"
Storyboard.TargetProperty="Border.Opacity"
RepeatBehavior="Forever"
AutoReverse="True"
From="1"
To="0.0"
Duration="0:0:1" />
UPDATE >>>
Ahhhhh, my bad... As the animation is defined inside the Border, there is no need to reference it, just use Opacity:
<DoubleAnimation Storyboard.TargetName="AnimatedBorderBrush"
Storyboard.TargetProperty="Opacity"
RepeatBehavior="Forever"
AutoReverse="True"
From="1"
To="0.0"
Duration="0:0:1" />