You can’t be forced to do anything you feel is unsafe. SFety assessments are personal and subjective, so you may not feel safe delivering in the dark, while others do. There are so many factors that contribute to this, but in the end a supervisor can’t force you to do it. Exercise tour right to refuse, end of story.
Sometimes I’ll finish up my route in the dark, but not when there’s any ice or snow or rain on the ground. I womt deliver someone else’s trip in the dark because I can’t fully assess the safety of my workspace in the dark with a headlamp.