gistfile1.txt

        private void Main_OnLoaded(object sender, RoutedEventArgs e)
        {
            var thread = new Thread(() => TextThread("Testing..."));
            thread.Start();
        }

        private void TextThread(string text, Brush brush = null)
        {
            var brushFixed = brush ?? Brushes.Black;

            Dispatcher.Invoke(System.Windows.Threading.DispatcherPriority.ContextIdle, new Action(() => AddLine(text)));

            for (var opacity = 0.01f; opacity <= 1f; opacity += 0.01f)
            {
                Thread.Sleep(1000);
                var newBrush = brushFixed.Clone();
                newBrush.Opacity = opacity;
                Dispatcher.Invoke(System.Windows.Threading.DispatcherPriority.ContextIdle, new Action(() => ChangeLine(newBrush)));

            }
        }

        private void AddLine(string text)
        {
            if (!Block.Dispatcher.CheckAccess())
            {
                Dispatcher.Invoke(new Action(() => AddLine(text)));
            }
            else
            {
                Block.Inlines.Add(new Run(text));
            }
        }


        private void ChangeLine(Brush brush)
        {
            if (!Block.Dispatcher.CheckAccess())
            {
                Dispatcher.Invoke(new Action(() => ChangeLine(brush)));
            }
            else
            {
                Block.Inlines.Last().Foreground = brush;
            }
        }