<Canvas
      Margin="20" 
      Background="Beige"
      Height="100">
        <TextBlock
        Name="MyText"
        Margin="20" 
        Width="640" Height="100" FontSize="48" FontWeight="Bold" Visibility="Visible">
        This is my text
             <TextBlock.Triggers>
                <EventTrigger RoutedEvent="TextBlock.Loaded">
                    <BeginStoryboard>
                        <Storyboard>
                           <BooleanAnimationUsingKeyFrames 
                            Storyboard.TargetName="MyText" 
                            Storyboard.TargetProperty="Visibility"
                            Duration="0:0:10" FillBehavior="HoldEnd">

                            <DiscreteBooleanKeyFrame Value="False" KeyTime="0:0:0" />
                            <DiscreteBooleanKeyFrame Value="True" KeyTime="0:0:10" />
                          </BooleanAnimationUsingKeyFrames>      
                        </Storyboard>
                    </BeginStoryboard>
                </EventTrigger>
            </TextBlock.Triggers>
            </TextBlock>
        </Canvas>

<TextBlock
        Name="MyText"
        Margin="20" 
        Width="640" Height="100" FontSize="48" FontWeight="Bold" Visibility="Visible">
        This is my text
            <TextBlock.Triggers>
                <EventTrigger RoutedEvent="TextBlock.Loaded">
                    <BeginStoryboard>
                        <Storyboard>
                           <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(TextBlock.Visibility)" 
                                                          Duration="0:0:10" FillBehavior="HoldEnd" RepeatBehavior="Forever">
                                <DiscreteObjectKeyFrame KeyTime="0:0:0">
                                     <DiscreteObjectKeyFrame.Value>
                                        <Visibility>Visible</Visibility>
                                      </DiscreteObjectKeyFrame.Value>
                                </DiscreteObjectKeyFrame>
                               <DiscreteObjectKeyFrame KeyTime="0:0:5">
                                     <DiscreteObjectKeyFrame.Value>
                                        <Visibility>Hidden</Visibility>
                                      </DiscreteObjectKeyFrame.Value>
                                </DiscreteObjectKeyFrame>
                            </ObjectAnimationUsingKeyFrames>
                        </Storyboard>
                    </BeginStoryboard>
                </EventTrigger>
            </TextBlock.Triggers>
    
            </TextBlock>