Many were, but also there were indigenous tribes and people of European ancestry. She is likely to a mutt. Someone of African, Native and European ancestry.
While you often can tell someone's heritage by their skin color and other physical traits, you often can't. My nephew and niece have an African American father. A man with incredibly dark skin and that frizzy hair that is common. But my nephew has lighter skin than me and freckles. Nobody would think he's African American. My neice kind of looks African American, but you might also guess Mexican or Native American. And she is neither.