You are assuming the boy would have screamed when seeing a dark skinned person.  The post talks about someone in a big black garment, covered from head to toe, without anyone in the elevator being able to see its face.  People dress up in similar fashions at Halloween to get a frightened response from others.  The image of something hooded black without a face showing has always denoted something dark in the Western culture, even the Devil and the henchmen.  Jung would have called it a symbol of evil, of darkness.  
You are missing the point.  Most of us fear that which we do not know, and that which we are kept from knowing.  
There was a time when we used to see women in clear blue burkas in
footage that reached us from Afghanistan.  Somehow that felt very different from the black outfits we see today.  Still, in all instances the covering of a face should not have a place in our open society.