A pair of ladies where the blonde is kind, and the brunette is not so kind (if not outright evil).
This might fall under Tropes In Aggregate, but I have seen it enough times. Also, this came close to an alliterative title, but "benevolent" and "benign" were the the closest synonyms, and neither really fits the trope well. Rolling Updates
You have a pair of (usually) ladies, and you can tell the nice one because she is blonde, and the not nice (or evil) one because she is a brunette. Why this is so can have various reasons. Perhaps it's because Everyone Loves Blondes. In The Eighties, it might have just been follow the leader due the popularity of Joan Collins's role in Dynasty. For whatever reason, this is in a number of shows. Also, how good and bad the characters are can vary. The blonde could be a Messianic Archetype who's a Friend to All Living Things, or just moderately more kind than the brunette. The Brunette could be an outright monster, or just a bit more spoiled and selfish than the blonde. This often includes Light Feminine and Dark Feminine. Heck, this could even include a Dumb Blonde and Brainy Brunette. Also, if this is part of a Blonde, Brunette, Redhead trio, the redhead could be anything, as long as the blonde and brunette fit their roles. A Sub-Trope of Good Colors, Evil Colors, Good Hair, Evil Hair. Compare Hair of Gold, Heart of Gold, Eerie Pale-Skinned Brunette. Contrast Blondes Are Evil.
Examples:Anime and Manga