I think they didn't know what they were doing. She was meant to be a love interest for the Wayans brother, they had no clue there were different types of elves or that they might of caused some confusion with their casting (if they did know then they would of done something to explain it). The point isn't whether she was a drow or not, it's whether they even realized there are evil dark elves in the game. The movie was marketed as a D&D movie, you think somebody would of familiarized themselves with the game before they made it.