I've seen Blue Seed several times as it is the absolute favorite of the Empress. We own the whole series on DVD plus the movie.
From what I've seen around the Webiverse, the plot is based very loosely on some Japanese folklore. I've seen elements from Blue Seed pop up in other anime and some live-action films also.
On the down side, the english-language voice actress for Momiji (Amanda Winn Lee) should be forced into retirement with the amount of unnecessary screaming she did.