Since the era of Jim Crow, black people could be more outspoken -- to a point, particularly if they were female. To illustrate this, Sassy "Mammy" figures could scold the family they worked for and playfully berate their employers (to show that Blacks were not being oppressed). As the Civil Rights movement came up in the 60s, black people in media could be more outspoken. And because of feminism, the same thing applied for women. Combine these, and you get the sassy black woman. It started with the heroines of Blaxploitation movies, like Coffy and Foxy Brown (both played by Pam Grier), and continued into the 1980s.