I read a book series as a child (in the 1990s) that involved two friends, one of the friends had black hair and was the only one they knew with black hair. It is revealed in the book that he has black hair because he eats meat, which is forbidden in their society.
I also remember there is a cave system that is revealed to have fire-breathing salamanders in it. In a second book in the series the cave system is shown to connect to another continent. Also the story does not take place on Earth but is instead set in a different world with two continents that do not know each other exist until the events of the second book.