The blue spider lilies in Demon Slayer hold symbolic significance as they are often associated with death and the afterlife in Japanese culture. In the anime, the blue spider lilies are shown in scenes related to tragic events or the presence of demons, emphasizing themes of loss, sacrifice, and the supernatural elements present in the series. Additionally, the blue spider lilies serve as a visual motif to foreshadow and highlight the dangerous and eerie atmosphere of the Demon Slayer world.