Seems to me unrealistic, that not one case could be found. Certainly nothing to do with "whiteness" and "blackness" if one thinks back 150 or so years ago.
Race these days is not all that clearly defined in mixed race people. From Wiki. Non-Hispanic Whites are a subset of White Americans, the other being White Hispanic and Latino Americans.
Would the rape by a Latino male of a woman with the ancestry of the woman below be "white sexual assault on a black female"? Not at all unthinkable. She's Soledad O'Brien. She works for CNN, and her mother is Black-Latino.
There's not much on the internet about white on black rape, but this passage caught my attention:
Because interracial rape is now overwhelmingly black on white, it has become difficult to do research on it or to find relevant statistics. The FBI keeps very detailed national records on crime, but the way it presents rape data obscures the racial element rather than clarifies it. Dr. William Wilbanks, a criminologist at Florida International University, had to sift carefully through the data to find that in 1988 there were 9,406 cases of black-on-white rape and fewer than ten cases of white-on-black rape. [320] Another researcher concludes that in 1989, blacks were three or four times more likely to commit rape than whites, and that black men raped white women thirty times as often as white men raped black women. [321]
It's from a book by Jared Taylor, Paved with Good Intentions: The Failure of Race Relations in Contemporary America (New York: Carrol & Graf, 1992), 92-93.
He's one of the founders of American Renaissance, smart, capable, biased, and extremely unlikely, from what I've read of his, to falsely make such claims.
FWIW.