TextBlock color in default style / ControlTemplate - wpf

Question

I have a default GroupBox style in which I want to make the default header font color blue. I can't seem to get the color to apply when set from within the ControlTemplate in the style. Since it's just using a ContentPresenter for the header and that doesn't have a Foreground property, I'm setting TextElement.Foreground (I've also tried TextBlock.Foreground) on the ContentPresenter, which as I understand it should make all TextBlock children of the ContentPresenter have a foreground of blue. However setting this has no effect.
I've also tried adding a resource to the ContentPresenter to override the default style of TextBlock to give it a blue foreground, which didn't help either. If I set the Foreground property on the GroupBox itself, every TextBlock including both the header and the content of the GroupBox gets its color set to that, but I only want to change the color of the header. I don't want to give the header an explicit HeaderTemplate because I want the user to be able to put whatever content they want in the header, I just want that content to have a blue foreground.
<Style TargetType="{x:Type GroupBox}">
<Setter Property="BorderBrush" Value="{DynamicResource TabItem.Selected.BorderBrush}"/>
<Setter Property="BorderThickness" Value="2"/>
<Setter Property="FontSize" Value="18"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type GroupBox}">
<Grid SnapsToDevicePixels="true">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="6"/>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="6"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
<RowDefinition Height="6"/>
</Grid.RowDefinitions>
<Border BorderBrush="Transparent" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.ColumnSpan="4" Grid.Column="0" CornerRadius="5" Grid.Row="1" Grid.RowSpan="3"/>
<Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Grid.ColumnSpan="4" CornerRadius="5" Grid.Row="1" Grid.RowSpan="3">
<Border.OpacityMask>
<MultiBinding ConverterParameter="7" Converter="{StaticResource BorderGapMaskConverter}">
<Binding ElementName="Header" Path="ActualWidth"/>
<Binding Path="ActualWidth" RelativeSource="{RelativeSource Self}"/>
<Binding Path="ActualHeight" RelativeSource="{RelativeSource Self}"/>
</MultiBinding>
</Border.OpacityMask>
</Border>
<Border x:Name="Header" Grid.Column="1" Padding="3,1,3,0" Grid.Row="0" Grid.RowSpan="2">
<ContentPresenter TextElement.Foreground="Blue" ContentSource="Header" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</Border>
<ContentPresenter Grid.ColumnSpan="2" Grid.Column="1" Margin="{TemplateBinding Padding}" Grid.Row="2" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>