I'm pretty sure this white-on-black rape vs black-on-white rape statistic comes from the 1999 Department of Justice Uniform Crime Report which was still recently accessible on the fbi.gov website. (Not sure it is still there).
CURIOUSLY, (on a different topic here) although Latinos are classified as "latinos" in the Hate Crime "victim" statistics, there is NO classification for "latino" in the Hate Crime perpetrator category. When it comes to perpetrators, "latinos" are lumped in with "whites." There is no such thing, according to the UCR (uniform crime report) of ONE SINGLE hate crime perpetrated by a latino. I saw this GLARING anomaly myself on the fbi.gov website less than a year ago! It is unbelievable. Wonder what the motive is behind this convenient "oversight." Hmmmmmmm.
As per my post above: I asked a latino friend of mine a couple months back why the FBI might classify latinos as "white" in their hate-crime tables. He responded that many latinos are of Spaniard blood, so regardless of language and culture they would still be "white."
HOWEVER, as to why they would conversely be called "latino" in the victim category on the very next page of the report he had no response...
On that point, I went to http://www.rainn.org/ "the nation's largest anti-sexual assault organization". They have about every rape statistic available, including the race of the victims. I could find no stats on the race of the perpetrators.