Unfortunately, black women have had a harder time getting their work published and being treated with respect. Like all women, their work is taken seriously when it's in the genre of women, but not in the overall genre of literature. Their work is qualified by the gender and the race of the writer--thereby minimizing the total contribution to the field of literature.
However, this is generally speaking and when you talk about the actual contribution rather than the perception that changes dramatically -- and black women's contribution is fully recognized, as witnessed by the success of Alice Walker, Toni Morrison, Terry Tempest Williams, Maya Angelou and dozens of other black women whose books have made the bestseller lists and even won Pulitzer Prizes. As for the black women as subjects, with the exception of the writers mentioned above, images of black women are too few and not varied enough.
I hope this helps.