I don't think that's really an issue at my school. Well first of all, there are either NO black women who join NPC sororities or there are VERY few. But there are many people of Asian and Hispanic descent in the NPC sororities. I myself am half Puerto Rican and I just joined Sigma Sigma Sigma, and no one criticizes me for it
And if a black woman wanted to join, I don't think she would encounter any issues.