Blue Spider Lily

The Blue Spider Lily is a mythical flower, said to look like a Spider Lily with blue colors instead of red. It has been described as an important ingredient of a medicine for Muzan Kibutsuji's treatment by his doctor during the Heian Period.

As discovered by Aoba Hashibira, it blooms only 2 or 3 times a year.

History
The Blue Spider Lily was an important base ingredient for a medicine created by the doctor who treated Muzan's deadly disease (which would supposedly kill him before he reached the age of twenty). However, frustrated by the worsening condition of his illness, the doctor was killed by Muzan before he could finish the medicine. It was then that Muzan realized the medicine was actually working; he obtained a strong body, but due to the medicine being a prototype, he consequently could not walk into the sunlight and craved human flesh, turning him into a Demon. Muzan deeply regretted killing the doctor as he realized his life-saving treatment worked and that he had no idea how to finish his treatment by creating the medicine without the Blue Spider Lily. As a result, he unsuccessfully searched for the flower for centuries in hopes of using it to complete the treatment and give himself a perfect immortal body.

Over a hundred years after Muzan's death, botanist Aoba Hashibira discovered that the Blue Spider Lily only blooms during the day two or three days a year. This also explains why Muzan never found one, as he would have been unable to go out while the sun was out. Unfortunately, due to a mistake, all the samples of the flower he had were destroyed. Though this may have been a blessing in disguise, as this makes it unlikely that anyone will ever be turned into a Demon again using it.

Trivia

 * The Blue Spider Lily is a flower that does not exist in reality. However, the Red Spider Lily is an important flower in Japanese folklore, believing to guide the dead to their next reincarnation. They also are believed to grow upon the shore of the Sanzu River, the river of the dead in Japanese folklore, and all who die must traverse the width of it. It is likely the author intends for the Blue Spider Lily to represent the antithesis of the Red Spider Lily, which would be damnation.