Hohoemi no Bakudan, by Mawatari Matsuko (??? can't recall the exact spelling of her name)
One of my all-time faves, on every single playlist I know. They did have several closing themes, beginning with Homework ga Owaranai, by the same artist---and her Daydream Generation, which was the final closing theme. In fact the closing themes were ALL good, all of them are on my playlists.
Too bad Flowers of Blood is gone. That site listed every theme and every artist, with lyrics in both Japanese and English.