Miss Emily is black person... does it want to indicate anything or have especial connotation?

I agree with the previous editor.  Miss Emily's father was a Captain in the confederate.  She has had the high regard of her town's people for many years.  Had she been black her life during the time of her youth would have been very different. 

Now if Miss Emily were a black woman it would stand to reason that the townspeople would have been very suspicious of her.  They would not have gone to her home to remove her dead father.  They would have called upon the black community.  When she went to purchase the poison they would sent someone to investigate.  Unfortunately, the south was a very different place for black people.

"Example of a black woman was treated in the story "—he who fathered the edict that no Negro woman should appear on the streets without an apron."

Even Homer would not have given Miss Emily the time of day had she been black because this is how he sopke of black people in the story.

"The little boys would follow in groups to hear him cuss the niggers, and the niggers singing in time to the rise and fall of picks."