Have you asked your optometrist?
I believe that it is quite normal and , now that Madeleine McCann is back in the News, I remember the journalists pointing out that she too, has a line in one of her eyes. You can see it in the photos in the Press.
I would say, don’t worry but if you are worried, then see your Optometrist.
Good luck
If you have always had the black line then dont worry about it. If it has only just appeared then see your doctor or optician.
If you have always had it it may be an extremely mild form of Coloboma, but if that is the case it wont get worse.
I’m not a doctor or medical professional but I’d say it’s a simple “iris freckle” and not anything you need to worry about. For your own piece of mind, ask your optician about it next time you visit the eye doctor. Also, search the internet for “ectropion uvie” because it might be that. Again these aren’t problems to worry about. Let us know what you find out. Your post helps other people coming to this forum in search of answers to questions like yours.