RenderLoop.Run(form, () =>
{
    SharpDX.ColorBGRA color1 = new ColorBGRA( 0.0f, 0.0f, 0.0f, 0.0f );
    SharpDX.ColorBGRA color2 = new ColorBGRA( 1.0f, 1.0f, 1.0f, 1.0f ); // <-- text alpha should be different from 0.

    device.Clear(ClearFlags.Target, color1, 1.0f, 0);
    device.BeginScene();


    // Make the text boucing on the screen limits
    if ((fontDimension.Right + xDir) > width)
        xDir = -1;
    else if ((fontDimension.Left + xDir) <= 0)
        xDir = 1;

    if ((fontDimension.Bottom + yDir) > height)
        yDir = -1;
    else if ((fontDimension.Top + yDir) <= 0)
        yDir = 1;

    fontDimension.Left += (int)xDir;
    fontDimension.Top += (int)yDir;
    fontDimension.Bottom += (int)yDir;
    fontDimension.Right += (int)xDir;


    // Draw the text
    _sprite.Begin(SpriteFlags.SortTexture | SpriteFlags.AlphaBlend); // <-- alpha blend flag should be specified for sprites with transparency
    font.DrawText( _sprite, displayText, 20, 20, color2);
    _sprite.End(); 

    device.EndScene();
    device.Present();
});