Symbolism, Imagery, Allegory
When Maura and the other house psychics do readings for their customers, they often pull out their trusty old tarot cards. One card in particular is important to Blue—it's the page of cups, which is a card that she's been told is her card. The girl on the card even looks like her.
The page of cups card represents Blue's sealed fate. Her aunts and mother have predicted that she's destined to kill her true love if she kisses him, and no matter what, it's going to happen. When Gansey comes in for his reading, the page of cups card shows up again just to prove to Blue that she's destined for this:
When she offered the cards to him, Gansey turned his face away like he was pulling a raffle winner. His fingers grazed the edges of the cards, contemplative. He selected one, then flipped it over to show the room.
It was the page of cups.
He looked at the face on the card, and then at Blue's face, and Blue knew that he'd seen the similarity. (15.122-124)
Even though at the time, Blue is completely not into Gansey and hates everything that he stands for, the page of cups card shows her that they're still interconnected somehow. Whether she likes it or not, her fate is somehow interwoven with Gansey's. The cards don't lie.
Gansey even recognizes "the similarity," though he doesn't know exactly why their fates are tied. But that's the thing about fates: Whether you understand them or not, they still have to run their course. And in this way, the card is a reminder of this dynamic in the story.