I had a problem with a single, slim, and kind of blurred looking darker line 1/4 to 1/3 of the way down my screen. I would describe it looking almost like the screen had an indentation or fold more than anything else since the picture underneath seemed fine.
I do have xfinity so I tried many of the solutions here. None of them worked and I was starting to get concerned. Being a bit of an extreme troubleshooter I dug into TV settings.
I noticed that when I changed various brightness levels the line would move up and down depending on the brightness, with brighter settings pushing it higher but never eliminating.
If your Samsung TV has an eco friendly setting to adjust brightness based on ambient light, try disabling that setting. It removed the line I was seeing. Seems like a bug and not a feature in this case. That faint line was driving me crazy! I can’t understand why that was the cause. It was almost as if it was applying a darkening gradient over the entire picture to cause it to emit less light.