When Dr. Mills wants to keep Cussy in the hospital, he threatens her autonomy and reminds her of her powerlessness against the will of others. He knows that her Blue skin isn’t a sign of illness, but he can use this as a rationale to keep her, and most other people (ignorant of her color’s true cause) would believe him. In this moment, Doc protects Cussy, earning her gratitude. Evidently, while he’s happy to blackmail Cussy into participating in research for his own interest, he’s unwilling to extend the same benefits to Dr. Mills. While he notes the similarity between the discrimination that Cussy Mary and Dr. Mills both face (as so-called “colored” people), Doc can threaten and rough up Dr. Mills because the power of white people in racially divided society of the early 20th century protects him.