The closest match I could make between this phenomenon and something that has been reported in the literature are the so-called sunset and sunrise shadow bands. In my copy of William Corliss's Handbook of Unusual Natural Phenomena published in 1983 by Anchor Books, he describes published reports in a journal called the Journal of the Optical Society of America 1945, vol. 35 p. 736, a discussion of these shadow bands.
Apparently, they are caused by diffraction of light from the setting of rising Sun, through a distant mountain range. The bands can be several miles wide, oriented north south, but traveling eastward across the sky in 30 seconds or less as the Sun dips below the horizon. These are not fixed, long lived features, and they only occur around the time of sunrise/sunset. I know of no reports or discussions to suggest that they are opaque to background stars seen in the twilight sky.
The particular observation that prompted this question was one made in Canada, and apparently at night when many stars could be seen, but not through the dark band. The north-south orientation seems to be correct for shadow bands from the setting/rising sun, but the other particulars do not match up very well. There are no astronomical phenomena (extra terrestrial ones) that could cause this effect so it must have been local to where the observers were located. This suggests, perhaps, an atmospheric phenomenon such as a cloud band of some sort. I have seen squall lines pass over Boston traveling west to east along a north-south axis that were very dark and narrow. Seeing such a thing by night would have resulted in a rapidly moving dark band of obscuration. Beyond this, I cannot offer much of an explanation.
As a personal comment, however, I hope that you enjoyed the display. It's lovely to know that Nature can surprise us with novel phenomena which, although covered by natural laws, are like the rainbow still spectacular and awe inspiring. Be it a bright meteor, Sun Dogs and haloes, or hurricanes and lightning, nature seems to be constantly inventive and all we can do is sit back and enjoy the view.