Attached property to update style trigger on event

I'm trying to use an attached property to trigger a style change on a UIElement when an event has fired.

Here is the case scenario:

A user sees a TextBox, and focuses then unfocuses it. Somewhere in an attached property it notices this LostFocus event and sets a property (somewhere?) to say that it HadFocus.

The style on the TextBox then knows that it should style itself differently based on this HadFocus property.

Here's how I imagine the markup to look...

<TextBox Behaviors:UIElementBehaviors.ObserveFocus="True">
<TextBox.Style>
    <Style TargetType="TextBox">
        <Style.Triggers>
            <Trigger Property="Behaviors:UIElementBehaviors.HadFocus" Value="True">
                <Setter Property="Background" Value="Pink"/>
            </Trigger>
        </Style.Triggers>
    </Style>
</TextBox.Style>

I've tried a few combinations of the attached properties to get this working, my latest attempt throws a XamlParseException stating "Property can not be null on Trigger."

    public class UIElementBehaviors
{
    public static readonly DependencyProperty ObserveFocusProperty =
        DependencyProperty.RegisterAttached("ObserveFocus",
                                            typeof (bool),
                                            typeof (UIElementBehaviors),
                                            new UIPropertyMetadata(false, OnObserveFocusChanged));
    public static bool GetObserveFocus(DependencyObject obj)
    {
        return (bool) obj.GetValue(ObserveFocusProperty);
    }
    public static void SetObserveFocus(DependencyObject obj, bool value)
    {
        obj.SetValue(ObserveFocusProperty, value);
    }

    private static void OnObserveFocusChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)