You probably already know that a Silverlight application inserted into a web page has the possibility to interact with the wrapping HTML and vice versa i.e. a button in the HTML could cause some sort of interaction with the Silverlight application, or a button contained in the Silverlight application could cause a table in the HTML to be updated.
So if this "black box" testing means you want to test how well your Silverlight application is interacting with the outside world, I recommend you focus on its interaction with the HTML part of the web application.
Testing things like Printing or Network really is testing the browser itself and it's code, which ultimately is Microsoft code... something you have no control over. Automated testing needs to focus on the things you have control of, which is your code. Don't spend your time testing someone third party code.