Blackie is also a slur used in both
the north and
the south (of the USA) to describe
black people-- most often men. Since it is like a name for a
dog it is
pejorative and pretty insulting.
There was a brand of
shoe polish sold untill the 70s called “blackie” –this alone would not have been so bad if the tin did not depict a
monkey-faced “
negro” with
pitch-black skin.