I'll bet at least 50% of the searchers are looking for porn.
So even if Google had infinite resources, its choices are: (1) act as a moral authority and deny pornographic contents to those who are looking for them, or (2) show pornographic results to people searching for innocent stuff.
...Or maybe (3) built a perfect profile of every user and just show them what they want. But do we want to go there?
(You might be thinking that "black girls" is such an innocent term that Google should be able to determine that it's non-porn, but then the question merely shifts to less popular queries.)