"The Blue Plague" is the only time in the cartoon series that a musical number is performed, as in this case by Gargamel.
The villain song Gargamel sings is a recycled version of a villain song featured in two of Hanna-Barbera's previous works, Oliver and the Artful Dodger: Part 1 (1972) and Yogi's First Christmas (1980), the backing track the same and the lyrics more resembling the 1972 original.