In French, the term "papillons noirs" is a 19th-century metaphor for dark thoughts , worries, or melancholy. Lyrics Highlight:
The term is famously used in the poem (Night) from Albert Giraud's cycle Pierrot Lunaire (1884), later set to music by Arnold Schoenberg.
This is currently the most popular reference. It is a French series (available on Netflix in many regions) about a struggling novelist, Adrien Winckler, who is hired to write the memoirs of an elderly man named Albert Desiderio.
I can provide more details on the thriller plot , the full lyrics of the Gainsbourg song, or the symbolism in poetry. De sinistres papillons noirs | LiederNet