IsFocusedなしでvalueがない場合はプレースホリダーのように表示するメモ
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type TextBox}}, Path=Text}" Value="" />
<Condition Binding="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type TextBox}}, Path=IsFocused}" Value="False" />
</MultiDataTrigger.Conditions>
<Setter Property="Foreground" Value="#33000000" />
<Setter Property="Visibility" Value="Visible" />
</MultiDataTrigger>
参考URL