have you tried debugging your application ?
In your case at the end of MainPage constructor you are filling your list box which eventually makes call to ListBox1_SelectionChanged method and the reason for the screen blank may be because of the operation performed side SelectionChanged method.
double check wc.DownloadStringAsync doesnt blank your page.
it's worth to note that
ListBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
method gets called when ever you try to add a new item to the list or you change the item selection.