The xamTextEditor contains a TextBox in its template when it is in edit mode. By default the wpf TextSelection calculates the CaretBrush based on the Background property of the hosting control (the TextBox in this case). Since the background of the xamTextEditor should be visible, the Background of the TextBox within its edit template is set to Transparent. When the TextSelection calculates the caretbrush it ignores the alpha and so the background is seen as white (transparent is 0x00ffffff) so it calculates that it should use Black.

In CLR 3, the only way to handle this would be to retemplate the xamTextEditor. Assuming the background is a solid non-alpha color then binding its background to that of the xamTextEditor should handle the issue. You can use the default xaml we include in the DefaultStyles directory as a starting point. You have to be careful if you are dynamically changing the Theme property as their are different templates used by the element depending on the theme chosen. Another possibility might be to derive from xamTextEditor but again with theming you have to be careful since the local style is based upon the class type which in this case would be your derived type. Something like the following should work.

 public class XamTextEditorEx : XamTextEditor
 {
  public XamTextEditorEx()
  {
   this.SetResourceReference(StyleProperty, typeof(XamTextEditor));
  }

  protected override bool ValidateFocusSite(object focusSite, out Exception errorMessage)
  {
   TextBox tb = focusSite as TextBox;
   if (null != tb)
    tb.SetBinding(TextBox.BackgroundProperty, new Binding { Path = new PropertyPath(XamTextEditor.BackgroundProperty), Source = this });
   return base.ValidateFocusSite(focusSite, out errorMessage);
  }
 }

In WPF4, MS added a CaretBrush property to the TextBox so you would just have to add a style for TextBox to its resources and set the CaretBrush there. e.g.

    <StackPanel>
        <StackPanel.Resources>
            <Style TargetType="TextBox">
                <Setter Property="CaretBrush" Value="White" />
            </Style>
        </StackPanel.Resources>
        <igEditors:XamTextEditor Text="XamTextEditor" Background="Black" />
        <igEditors:XamTextEditor Text="XamTextEditor" Background="Red" />
    </StackPanel>