Why are black women dispropotionally represented? Excellent question. I would suggest that it's because they have less access to contraceptives given the fact that they earn about 17% less than white women. They probably are less likely to have insurance as well.

Ironically pro-life attacks on Planned Parenthood will increase the number of abortions. 

http://www.pewresearch.org/fact-tank/2016/07/01/racial-gender-wage-gaps-persist-in-u-s-despite-some-progress/