WPF 圆角背景渗出

我第一次尝试 WPF - 我有一个简单的表单，其中包含一个为内联帮助定义的弹出窗口.我正在使用圆角，由于某种原因，黑色背景在角落里渗出.我不明白是哪个元素导致了问题.

I'm making my first foray into WPF - I have a simple form with a popup defined for inline help. I'm using rounded corners, and for some reason a black background is bleeding through around the corners. I don't understand which element is causing the problem.

替代文字 http://www.awbrey.net/rounded.jpg

我认为这是我没有看到的非常明显的东西.这是我正在使用的 XAML:

I assume it's something blindingly obvious which I'm just not seeing. Here's the XAML I'm using:

<Window x:Class="Consent.Client.SubjectNumberEntry"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" FontSize="24"
    Title="SubjectNumberEntry" WindowStyle="None" WindowState="Maximized"
        xmlns:h="clr-namespace:Consent.Client" KeyDown="windowOuter_KeyDown" Background="White" Name="windowOuter" AllowsTransparency="true" Loaded="Window_Loaded">

    <StackPanel Height="400" DockPanel.Dock="Top" HorizontalAlignment="Center" VerticalAlignment="Center" Margin="10">
        <StackPanel Height="60" Orientation="Horizontal" VerticalAlignment="Center">
            <TextBox Name="txtSubjectNumber" Margin="10" Width="400" KeyDown="txtSubjectNumber_KeyDown" h:HelpProvider.HelpString="Enter the subject identifier, or scan their wristband">
                <TextBox.ToolTip>This is a textbox</TextBox.ToolTip>
            </TextBox>
            <Button Name="btnEnter" Margin="10" Width="100" Click="btnEnter_Click">Enter</Button>
            <Button Width="50" Name="btnHelp" Margin="10" Click="btnHelp_Click">?</Button>
            <Button Width="50" Name="btnExit" Margin="10" Click="btnExit_Click">Exit</Button>


        </StackPanel>
        <Label Name="lblValue" Margin="10"></Label>


        <Popup Placement="Bottom" HorizontalAlignment="Center" VerticalOffset="10" MouseDown="popHelp_MouseDown" PopupAnimation="Fade" Name="popHelp" PlacementTarget="{Binding ElementName=txtSubjectNumber}">
            <Border Padding="10" Margin="10" BorderBrush="CornflowerBlue" BorderThickness="1" CornerRadius="10" Background="CornflowerBlue">
                <TextBlock FontSize="12" Background="CornflowerBlue">This is the content of the help box.</TextBlock>
            </Border>
        </Popup>

    </StackPanel>


</Window>