I want to make TextBlock like this

But,
aTextBlock.Begi
You need to use ThicknessAnimation for this. DoubleAnimation is for properties of Double type.
<TextBlock.Triggers>
<EventTrigger RoutedEvent="TextBlock.Loaded">
<BeginStoryboard>
<Storyboard>
<ThicknessAnimation
Storyboard.TargetName="TestB"
Storyboard.TargetProperty="(TextBlock.Margin)"
To="2"
Duration="0:0:1" AutoReverse="True"
RepeatBehavior="2" />
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</TextBlock.Triggers>