Why is my WPF Popup black and how do I get it positioned properly?

A quick WPF tip:

Problem: I was trying to create a cool OS X style semi-transparent round cornered popup but no combination of setting Background=Transparent or Opacity<1 would give me anything but a big square cornered black box in the wrong part or the screen.

Solution: You need to set the AllowsTransparency property on the Popup to True, and set the PlacementTarget and Placement properties to control the position the Popup opens in.

<Popup x:Name="RecipeSourcePopup"

               PlacementTarget="{Binding ElementName=MainStackPanel}"

               Placement="Center" AllowsTransparency="True"

               HorizontalAlignment="Center" VerticalAlignment="Center" >