The "dark speck" that Elisa notices at the end of the story as she drives into town with her husband for dinner refers to the dirt that she gave the peddler. This of course triggers off a kind of epiphany in Elisa as she is forced to realise that the connection she felt she had forged with the peddler was based on nothing more than his feigned interest. Note how Elisa responds to seeing this "dark speck":
She tried not to look as they passed it, but her eyes would not obey. She whispered to herself sadly: "He might have thrown them off the road. That wouldn't have been much trouble, not very much. But he kept the pot," she explained. "He had to keep the pot. That's why he couldn't get them off the road."
The way in which this dark speck impacts Elisa so greatly only serves to reinforce what a pitiful figure she is. She is so intensely alone that she made herself believe she had forged a meaningful relationship with a complete stranger, whilst being blind to his indifference all along. The "dark speck" therefore only serves to underline how completely alone Elisa is in her life, and how isolated, without anybody who can realy understand her.