Of Blood and Fire by Jennifer Jenkins

Natalia D’Angelo has spent the last seven years planning for the day she can free both herself and her parents from the brutal clutches of slavery and return back to Valencia to find her little brother, Antonio. To everyone around her, even her best friend, she appears to be just another half-starved young man in the slave camp. This is only one of her lies and the key to her survival. But if someone was to see her chest binding or have other reasons to suspect her true nature, her mother’s spell will break, along with the illusion that she isn’t a young man at all.When a handsome quarry worker discovers Natalia’s true identity, and learns her own father might be working to help the slave master, Natalia turns to a darkness within herself that she’s willed away for years–a magic of blood and fire that might be her greatest resource and most powerful enemy.