Without knowing further details of the passage or book, we can only guess. With that being said, the author may have used the simile to describe how as the night progresses, it gets darker, as if the security blanket was covering the area.
Beacause the author is trying to describe how night time is really like, a pitch black secrity blanket is very dark compared to the night time. So the author is comparing both of them to the same topic.
