Here is the trick for putting your Silverlight component into the HTML host page transparently. When you follow the steps the Silverlight plug-in background will be same as your html page background.
You need to only add the following parameters to your object tag :
<param name="background" value="Transparent" />
<param name="pluginbackground" value="Transparent" />
<param name="windowless" value="True"/>
Also try to make Silverlight's layout root background as transparent. Cheers!! J