The blonde girl has actually been acting smitten with the dark-haired girl for a while, even though she clearly hasn't understood her own feelings. You just have to go back and look and you can find plenty of signs. And yes, she's also been trying to get into the boy's pants since the beginning. So she can find both males and females attractive, and she just didn't realize the female part until now. Welcome to teenagers figuring out their sexuality. Sometimes it's confusing.