Not sure if I've got your requirement correctly,
but is this what your looking for?
If so you don't need the link in my comment tbh. You could just go with:
<Storyboard x:Key="SomeStoryboard"
AutoReverse="True"
RepeatBehavior="Forever">
<DoubleAnimationUsingKeyFrames Storyboard.TargetName="textBlock"
Storyboard.TargetProperty="(UIElement.RenderTransform).(TransformGroup.Children)[0].(ScaleTransform.ScaleX)">
<EasingDoubleKeyFrame KeyTime="0:0:1"
Value="-1" />
</DoubleAnimationUsingKeyFrames>
</Storyboard>
...
<TextBlock x:Name="textBlock"
HorizontalAlignment="Left"
RenderTransformOrigin="0.5,0.5"
Text="Some random text">
<TextBlock.RenderTransform>
<TransformGroup>
<ScaleTransform />
<SkewTransform />
<RotateTransform />
<TranslateTransform />
</TransformGroup>
</TextBlock.RenderTransform>
</TextBlock>