Boring. The cute cunning girl should be the one with dark hair.
No fictional character in their right mind would ever trust a fictional girl with light hair as far as they can throw them. Especially if they are cute.
Making the cute cunning girl light-haired immediately breaks the spell of being immersed in a story, because it's done to death so much that it clashes with a person's ability to believe that other characters can still be fooled that way.
That's why the cute cunning girl should be a dark-haired girl. People don't expect it because if anything cute dark-haired girls are always depicted as actually innocent.