Can't see contextmenu on screen windows phone

ContextMenu cm;
TextBlock tb;
private void BuildTB()
{
    cm = new ContextMenu();
    cm.Background = new SolidColorBrush(Colors.Black);
    MenuItem menuItem = new MenuItem()
    {
        Header = "Edit",
        Foreground = new SolidColorBrush(Colors.White),
        Background = new SolidColorBrush(Colors.Black)
    };
    menuItem.Click += EditItem_Click;
    cm.Items.Add(menuItem);

    tb = new TextBlock();
    tb.Text = "My text block";
    tb.Hold += OpenCM;
    ContextMenuService.SetContextMenu(tb, cm);
}

private void OpenCM(object sender, System.Windows.Input.GestureEventArgs e)
{
    TextBlock tb = (TextBlock)sender;
    ContextMenu cm = ContextMenuService.GetContextMenu(tb);
    cm.IsOpen = true;
}