Help with binding to parent property on a style

Hi,

I'm trying to set up a textblock style such that if text is empty, the textblock displays some default information.

I'm currently having some trouble setting up RelativeSource, as I understand it and have seen elsewhere. I've tried a number of solutions found on forums, but no luck as of yet.

<TextBlock Grid.Row="5"
                                   Name="test"
                                   TextWrapping="Wrap"
                                   Text="{Binding Context}"><TextBlock.Style><Style BasedOn="{StaticResource labelTextBlockStyle}" TargetType="TextBlock"><Style.Triggers><!-- Can't re-bind to text propery since it would cause an infinite loop--><DataTrigger Binding="{Binding Text.Length, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type TextBlock}}}" Value='0'><Setter Property="Background"><Setter.Value><VisualBrush Stretch="None"
                                                                 AlignmentX="Left"
                                                                 AlignmentY="Center"><VisualBrush.Visual><TextBlock Text="None"
                                                                       FontStyle="Italic"
                                                                       Foreground="DarkGray"></TextBlock></VisualBrush.Visual></VisualBrush></Setter.Value></Setter></DataTrigger></Style.Triggers></Style></TextBlock.Style></TextBlock>

When I use ElementName=test, it works as I expect it to. However, when I use the RelativeSource it never applies the DataTrigger. (The VisualBrush never appears.)

I'd like to keep it with some relative binding, as I would like to actually apply this style to several TextBlocks.

What am I doing wrong?

Thanks!