Using Staticresouce in Visual State causes Blend Designer to break: "Invalid attribute value Unknow for property Background" - xaml

Question

I'm trying to change the Background of a Grid by using a Visual State and a Staticresouce as the value in the Setter of the Visual State. It works just fine at Runtime but the designer shows the following error (which doesn't help me a lot):
Exception: Der Text zu diesem Fehlercode wurde nicht gefunden. (unknown error)
Stacktrace:
at Windows.UI.Xaml.Hosting.XamlUIPresenter.Render()
at Microsoft.VisualStudio.DesignTools.WindowsXamlDesigner.Views.WindowsUIXamlImageHost.RenderWorker()
at Microsoft.VisualStudio.DesignTools.WindowsXamlDesigner.Views.WindowsUIXamlImageHost.RenderScheduler.OnRender(Object object)
at Microsoft.VisualStudio.DesignTools.WindowsXamlDesigner.Views.WindowsUIXamlImageHost.RenderScheduler.b__26_0(Object o)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
Invalid attribute value Unknown for property Background.
InnerException: None
This is what I tried:
<Grid x:Name="grid">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="TestGroup">
<VisualState x:Name="TestState">
<VisualState.Setters>
<Setter Target="grid.(Panel.Background)" Value="{StaticResource BackgroundBrush}" />
</VisualState.Setters>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
</Grid>
<ResourceDictionary
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<SolidColorBrush x:Key="BackgroundBrush" Color="Black"/>
</ResourceDictionary>
<App>
...
<App.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="Resources/Test.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</App.Resources>
</App>

Answer 1

The current Blend is really buggy so I am not surprised if you see errors like this.
One workaround is, reopen your page that contains this resource in Blend. You should see a dialog popping up (see below image). Then select your Test.xaml file from the Available dictionaries dropdown and hit OK.
By doing so blend will generate a DesignTimeResources.xaml file under your project Properties and it should eliminate the error on the designer.
However, I also noticed that your <ResourceDictionary.MergedDictionaries> isn't wrapped within a <ResourceDictionary>. Shouldn't it be something like this instread?
<App.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="Resources/Test.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</App.Resources>
Update
This is indeed a Blend bug which even happens with built-in resources. To replicate, try building the project and then go to the States tab and select a VisualState, right after you will see the Blend Designer crashed. Note that I have Run project code in XAML Designer (if supported) turned off.
Currently the only way to remove the crash is to reopen the xaml file.