These don't seem to do it for me:
.RadChart1.Background = new SolidColorBrush(Colors.Blue);
.RadChart1.DefaultView.ChartArea.Background = new SolidColorBrush(Colors.Black);
The only way I have done it so far is to override the whole control template, and set the Background on the WrapPanel....which is very much over-kill. I must be missing something obvious. If you want to change the background color of a control, you usually just set the Background property...