In this scene, the narrator describes Boldwood as a “Daniel” in Bathsheba’s kingdom, making a biblical allusion:
Boldwood’s had begun to be a troublesome image—a species of Daniel in her kingdom who persisted in kneeling eastward when reason and common sense said that he might just as well follow suit with the rest, and afford her the official glance of admiration which cost nothing at all.
The biblical figure Daniel was noted for his refusal to conform to certain religious decrees in his era, including one that forbade praying in the direction of Jerusalem. Here, Bathsheba sees Boldwood, who is cold to her beauty, as her personal “Daniel," or someone who refuses to conform to the near universal admiration of her beauty.
Bathsheba’s pride and ego are on full display here. “Reason and common sense” demand that Boldwood adore her as everyone else does, she thinks. Crucially, this allusion makes clear what Bathsheba’s motivation is when she sends Boldwood the valentine. She is not merely teasing—she is getting back at him in a small way for withholding his attention. Consider how attention grabbing the valentine itself is, with its bold stamp and lack of signature. This allusion thus reveals Bathsheba's vanity, making it clear that she wants to be liked by people she doesn't even care all that much about.