Wow, what is this??? I created a black 400x800 px image and saved it to my phone and pulled up the image while I was in the dark and I could see it BUT mine is in a different spot (maybe 3/4 inch from left and a little over an inch from the top measured from LCD, not body). There was also a blotch or an angled shape to the top and left of the circle. The circle is about 1/4 inch I'd say.
I tried the sensor in that area and everything seemed to work normally.
Also, I created white, red, green, blue, mid-grey, and light-grey images to see if it was noticeable with any of those but it was not. Like you said, it is only noticeable if you are in the dark and look at a black image....it's like no light is coming through that area whereas the rest of the screen has some light even though it is a black image.
I need to get some sleep so will check back later to see if anyone else has insight. I also have that tech manual for this phone so maybe it will have something in there about it...I will look later. It would be nice if a lot of people do this test (as well as the other Lumias) and see what the result is so we can see if it is with all phones or if it is really an issue with just some.