Susan Coffey to paraphrase sparknotes, the "lady in black" represents widowhood, Edna is longing for independence and freedom, in Victorian times becoming a widow was the only socially acceptable way of gaining independence via freedom from marriage. "The lovers" represent Edna and Robert and the life they may have had if circumstances were different, the lady in black is juxtaposed with "the lovers" to implicate an inevitable failure of the relationship.
Hope For myself, I saw the lady in black and the two lovers as symbols for the tarot cards Death and the Lovers, respectively. Tarot was and is a part of the culture of New Orleans and something both Edna (the protagonist) and Chopin (the author) would have been exposed to in some form. I saw it as the lady in black signifying an end to a relationship (Edna's and Leonce's) and the lovers presenting an important choice about a relationship (Edna's and Robert's). This is, of course, merely my perception, but several points lead me to this fact. Firstly, they are never named, and almost never directly addressed. They have little, if any, personality, thoughts, or desires outside their roles in a story full of captivating characters. Secondly, they all appear before, during, or after key events frequently. Lastly, the story is set in and around New Orleans, a place renowned for its population of the occult.