Belladonna is a mundane (but toxic) plant with magical uses (DP2, JKR-W1).
- Essence of belladonna is part of the standard potions-making kit used by students (GF10).
In magical lore, aconite combined with belladonna was applied as a magical ointment by witches to make themselves fly.
"bella donna" = Italian "fair lady", "originating either from its usage as cosmetic for the face, or, more probably, from its usage to increase the pupil size in ladies" (Wikipedia).
In year four when Harry returns to Hogwarts, he has among his new school supplies essence of belladonna which Molly Weasley purchased for him in Diagon Alley.
"bella donna" means "fair lady" in Italian
It's possible that the mention of poisonous belladonna is foreshadowing for the first appearance of Bellatrix Lestrange, whom Harry sees in Dumbledore's Pensieve memory (GF30). When Harry sees her again at the Battle for the Department of Mysteries, Voldemort refers to her as "Bella" - either a term of endearment meaning "beautiful" in Italian, or perhaps a reference to her poisonous nature (OP36).
This is a plan which produces toxic berries and small, dark-colored flowers. It is not the same plant as Amaryllis belladonna aka the belladona lily.