It's not clear that "the base rates for violence among blacks is higher" is meant to be measured by convictions. I interpreted it to be based on sociological data, for example, and in that case there would be no feedback loop. Pinker didn't cite a source, unfortunately. A very quick stroll past Google Scholar 1 2 shows that a common source used is arrest data in the FBI's Uniform Crime Report. Plainly there are also important ways in which arrest data may be biased against blacks, but I'm hesitant to simply dismiss a finding based on that difficulty as I'm willing to bet that researchers in the field would have attempted to account for the difficulty.
The right kind of data would come from things like the National Crime Victimization Survey which collects data outside the criminal justice system. The base rate for the offender in a violent crime being black, is, according to that survey, lower than the probability that a given person arrested for a violent crime is black. So it looks to me that the evidence is long screened out by the time the case gets to the court room.
This thread is for discussing anything that doesn't seem to deserve its own post.
If the resulting discussion becomes impractical to continue here, it means the topic is a promising candidate for its own thread.