This is happening because the background of your PDF is transparent. When you save it as JPEG this will turn black for your PDF's. You should change the transparent pixels to a color with the ColorAlpha method:
foreach (MagickImage image in images) { image.ColorAlpha(new MagickColor("#fff")); image.Write(fileName.Substring(0, fileName.Length - 4) + " Page " + page + ".jpg"); page++; }