Why is the woman in this painting in the dark?
It's interesting that the parlor is more brightly lit and becomes the focus of the scene while she is off to the side and in the darker staircase area.
The title does provide a clue. Someone may be paying her a formal visit at her home. If a woman chose not to receive guests at that moment, she could tell her maid to tell the visitor that she was "not at home." She seems to be excusing herself from the scene and moving into the private, darker part of the house.
Yes, it looks as if she's hiding. But it's a great approach to portrait exactly this moment!
Indeed! We've probably all been "not at home" at some point when someone has stopped by.