Molly, a lot of horses with blue eyes on a white face tend to have eye problems that can eventually lead to blindness. I know here in the SW, the sun poses the greatest risk for this situation, so many people tattoo "eyeliner" to prevent this from happening. It acts much the way a football players black smears under their eyes does. It prevents sun glare/ /sunburning to the eye. Others may have a more defined reasoning, this is just a quick response, since it is something I have to watch on my little boy as well.