The first time you add a Silverlight 3 control which is supposed to have transparent portions to let your site design shine through you might notice that those sections are black and not the anticipated transparency.
Silverlight controls by default have a window that surrounds them and is set to black. Transparency of course takes a lot of resources to accomplish and as you would guess it is turned off by default. In order to get the desired effect you need to alter an existing parameter and add an additional one.
Existing Parameter: <param name="background" value="white” />
Changed To: <param name="background" value="transparent” />
Additional Parameter: <param name="windowless" value="true” />