Adding dark borders isen't something i'm a fan of (i'm assuming that's what you mean atleast).
It's true that with one eye cloused, you are likely to notice the contours of your nose and brow, usually as a dark border in the left of your field of view (assuming you are looking out of the right eye), but this is because the human eye has such a wide field of view, and for some reason, your brain seems to ignore the nose and brow when you have both eyes open, but not when you clouse one eye.
A screen does not have anywhere near the same field of view as the human eye, no real periphial vision, that's all lost here, so what you see out of the corner of your real eye, is happening off screen in a game.
Our periphial vision is allready beeing limited by quite a bit in a game, and thus, adding dark borders that should take place in the periphials, to even futher limit the limited screen realestate, well, that's not really making it more realistic, it's just making it much harder to see than it should be.
Of course, it can be argued that it does add some immersion, but i think it comes at too high a cost, the screen allready robs us of so much periphial vision, and this would just rob us of even more of it.