The latest version of RadControls that officially supports .Net 3.5 is Q2 2012 SP2. As for this particular case after research it appeared the CaretBrush property of the TextBox is available in the .Net versions 4.0 and above. However as a workaround you can set the Background of the TextBox to Black as the CaretBrush takes automatically the opposite color of the Background. So you could either extract the ControlTemplate and set the Background of the mentioned TextBox or I can suggest you another approach which sets the property in the code behind without using Implicit Styles.
If you have the following ComboBox:
You can set the property of the TextBox the following way:
var editableTextBox = (TextBox)
editableTextBox.Background = Brushes.Black;
Hope this will work for you.