 <p>WP8.1 map tile layers do not support opacity on their own. However you can add it by using a custom tile source. I have a code sample of how to do this here: <a href="https://code.msdn.microsoft.com/Adding-Opacity-and-WMS-cf6773f1" rel="nofollow">https://code.msdn.microsoft.com/Adding-Opacity-and-WMS-cf6773f1</a></p>