Bug ID: 502818

When customizing the style/template of a ChildWindow view in blend 3 the in-document style is applied to the child window control by way of this XAML:

<controls:ChildWindow.Style>

     <StaticResource ResourceKey="ChildWindowStyleBlack"/>

 </controls:ChildWindow.Style> 

When returning to Visual Studio 2010 the XAML parser throws an error and will not render the designer with the message:

Property 'Style' does not support values of type 'StaticResourceExtension'.

However removing that piece of XAML and setting the Style property of the child window control at the control definition like below works:

<controls:ChildWindow

           xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

           xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

           xmlns:controls="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls"

           xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d" xmlns:VMRSilverlight_ViewModel_Controls="clr-namespace:VMRSilverlight.ViewModel.Controls"

           x:Name="childAddSite" x:Class="VMRSilverlight.View.Popup.AddSite"

           Width="400" Height="500"

           Title="AddSite" Style="{StaticResource ChildWindowStyleBlack}">

So to see the design surface in Visual Studio 2010 B2 you must use the Style reference:
Style="{StaticResource ChildWindowStyleBlack}". If you leave the style reference in like this and run the silverlight application it will error out with the following exception:

Unhandled Error in Silverlight Application Cannot find a Resource with the Name/Key ChildWindowStyleBlack [Line: 8 Position: 34]

So, after the control is designed and tweaked in VS 2010 B2 you must remove the style reference that VS2010B2 likes and add back in the Style reference that Blend 3 put in. The app will now execute correctly.