// Create and configure a simple color animation sequence.  Timespan is in 1000ns ticks.
ColorAnimation colorAnimation =
    new ColorAnimation(Colors.Maroon, Colors.White, new Duration(new TimeSpan(1000000)));
colorAnimation.AutoReverse = true;
colorAnimation.RepeatBehavior = RepeatBehavior.Forever;

// Create a new brush and apply the color animation.
SolidColorBrush solidColorBrush = new SolidColorBrush(Colors.Black);
solidColorBrush.BeginAnimation(SolidColorBrush.ColorProperty, colorAnimation);

// Create a new TextEffect object. Set the foreground to the color-animated brush.
TextEffect textEffect = new TextEffect();
textEffect.Foreground = solidColorBrush;

// Apply the TextEffect to the entire range of characters.
textEffect.PositionStart = 0;
textEffect.PositionCount = int.MaxValue;

// Create a new text Run, and add the TextEffect to the TextEffectCollection of the Run.
Run flickerRun = new Run("Text that flickers...");
flickerRun.TextEffects = new TextEffectCollection();
flickerRun.TextEffects.Add(textEffect);

MyFlowDocument.Blocks.Add(new Paragraph(flickerRun));