I think you have touched upon a very fundamental part of the symbolism of this story. What fascinates me is that the story begins with lots of images of light and then as we move ever closer to the crushing epiphany that the narrator experiences, we have more images of darkness.
Key to the images of light is the presentation of Mangan's sister, the focus of the narrator's Romantic dreams:
She was waiting for us, her figure defined by the light from the half-opened door.
The light from the lamp opposite our door caught the white curve of her neck, lit up her hair that rested there and, falling, lit up the hand upon the railing.
Both of these images use light to present Mangan's sister as an almost angelic figure - a suitable receptacle for all the boy's desires and Romantic dreams.
However, as the narrator nears Araby, we see more and more images of darkness:
Nearly all the stalls were closed and the greater part of teh hall was in darkness.
The upper part of the hall was now completely dark.
It is entirely fitting then that the narrator should experience his epiphany about the vanity of his hopes and life in complete darkness, for he has had his illusions and hopes cruelly ripped away from him, and all but darkness remains:
Gazing up into the darkness I saw myself as a creature driven and derided by vanity; and my eyes burned with anguish and anger.
Images of light and darkness thus of course play a central role in establishing and supporting the theme of this excellent short story.