"The Black Hellebore" is not based on any "Johan et Pirlouit" ("Johan and Peewit" in English) comic story. It is an original story.
The instrument that Peewit plays is called a hurdy-gurdy.
Hellebore plants exist in the real world and are relatively harmless.
In this episode, Hellebore flowers put any person (or Smurf) who smells them into a trance, causing them to mindlessly follow orders. The effects of the Hellebore flower last one hour, regardless of whether on Smurfs or humans.
It is implied in this episode that Johan loses swords rather frequently.