Are you able to view layer information using your web browser?
You might find this blog post useful too: http://blogs.esri.com/Dev/blogs/silverlightwpf/archive/2009/08/24/Troubleshooting-blank-layers.aspx
I mean, if you copy-paste the service URL to Internet Explorer/FireFox/Google Chrome, do you see anything on this web page? The reason I ask is to narrow-down the cause of blank page by not using SL app and simply viewing information on the layer through your web browser. Please try to use Fiddler when you do this too.