1) For any more complicated styling scenarios it will be most appropriate if you use our implicit style mechanism to modify the default TrackBallInfoControl Style. Please follow this article for the purpose. You may do the changes to the default control template directly in theTelerik.Windows.Controls.Chart.xaml file in your project.
2) As for the Blend - in fact I should admit that it won't help you discover the default style of the TrackBallInfoControl, however you can easily do so by following the approach I wrote you about in 1)
the Telerik team