Could be what I heard called "mosquito noise", where that part of the picture looks busy, or something. Is it more noticeable from 12' vs 18'?
There is a setting on the TV called "video noise". The default setting is standard, and the other option is "low", I think. I toggled this setting while watching something, and it did make a difference in the picture, but it was with the whole picture, not just darks.
Maybe give that a try, if not already attempted.