attitudes
Up through World War II and into the '50s such attitudes as shown here prevailed. It's probably hard for many younger persons to believe that these portrayals were true to life.
As for casting: there's no way that Jean Crain could possibly be such a "pass for white" negro especially considering how her near relatives look. For her to pass for white her mother would've had to have been an octaroon at the most and her father 100% white. However, back then when this film was made, had they cast anyone who might pass for white, but who wasn't, there would've been an uproar about it. Attitudes were very constrained about these subjects.