That worked! Thanks! All I had to do was add this to my app.xaml:
xmlns:telerikMask="clr-namespace:Telerik.Windows.Controls.MaskedTextBox;assembly=Telerik.Windows.Controls.Input"
<Style TargetType="telerikMask:ExtendedTextBox">
<Setter Property="CaretBrush" Value="White" />
</Style>