*ahem* Tomboys are literally just girls who break the stereotypes we as a society have set out for them. If girls have muscle, if girls act masculine, if girls like video games or cars or working real hard at manual jobs, all of those things are tomboyish because society has depicted girls as people who should devote their lives to caring for their children and not stuff men are typically attracted towards.
Plus, there is a difference between blurring the line between gender and WANTING to blur the line between genders. There are actually conditions people have where they are females that act more like men because they produce a lot of testosterone. Remember Pulaski, the polish general who saved Washington's life? Yeah, he was a tomboy. Like, no seriously, it's been tested, he was a girl with the aforementioned condition, and was just raised as a man because he was raised in the 17th century.