To dream of calling someone a nigger may reflect frustration with a jealous spiteful person. Someone that can't do what they want and is getting back at you for it.
To dream of being called a nigger represents anger you feel from someone else because of your jealous spitefulness. You can't do something so you don't want other people doing it either.
For a black person to dream of being called a nigger may reflect a person or situation that you feel enjoys noticing itself being superior to you or not letting you feel good about yourself.