Lightsong Sees the Painting of the Red Battle
This is our first major clue (though a subtle one at the same time) that there might be something to the religion of the Iridescent Tones. Lightsong does see something in this painting that an ordinary person wouldn't be able to. A well-crafted piece of art, made by a person channeling the Tones and connected to them via Breath, can speak to a Returned. Now, in this case, it doesn't work quite like Llarimar says it does—Lightsong doesn't actually prophesy about the black sword in the way the priest thinks. In other words, Lightsong isn't prophesying that he'll see the Black Sword (Nightblood) in the day's activities.
Instead, Lightsong is seeing an image of a previous war, which is prophetic in that another Manywar is brewing—and in both cases, Nightblood will be important to the outcome of the battle.
The person Lightsong sees in the abstract painting is Shashara, Denth's sister, one of the Five Scholars and a Returned also known as Glorysinger by the Cult of the Returned. She is seen here in Lightsong's vision as she's drawing Nightblood at the battle of Twilight Falls. It's the only time the sword was drawn in battle, and Vasher was horrified by the result.
It's because of her insistence on using the sword in battle, and on giving away the secret to creating more, that Vasher and she fought. He ended up killing her with Nightblood, which they'd created together during the days they were in love—he married her a short time before their falling out. That marriage ended with him slaying his own wife to keep her from creating more abominations like Nightblood and loosing them upon the world.
Nightblood is part of a much larger story in this world. He's dropped casually into this particular book, more as a side note than a real focus of what's going on, but his own role in the world is much, much larger than his supporting part here would indicate.