Belladonna has poisonous alkaloids in all parts, causing it to be one of the worlds more toxic plants. Consequently, of course , it has long been used as medicine. The alkaloids are atropine, scopalamine, and hyocyamine, which are familiar compounds even in modern medicine. That stuff that dilates your pupils in eye exams? Could be this. It is most used as a pain medicine. It has also been used recreationally because of the hallucinogenic side effects, but of couse the main effect is too often death!
The flowers vary from yellow to purple. I kind of feel cheated that the ones in the garden weren't purple.