There is no way to get the navigation history, you can store it by yourself by listening the navigation service event NavigationService.Navigated (or Frame.Navigated for frame navigation).
private List<Uri> _navigationHistory = new List<Uri>();
void onNavigated(object sender, NavigationEventArgs e)
{
_navigationHistory.Add(e.Uri);
}
private Uri getBackUri()
{
return _navigationHistory.Count > 1
? _navigationHistory[_navigationHistory.Count - 2]
: null;
}