You know that a lot of the murderers are black because in many cases even if the case wasn't solved, the police have eyewitness accounts from, for instance, friends of the deceased, identifying the perpetrator as black.

When police simply won't show up to certain black neighborhoods, you would expect that the numbers would under-count, not over-count, the number of murders of black people by black perpetrators, since proximity is an important factor for crimes. (If it's a heavily mixed-race neighborhood, then it might be an under-count; hard to know.)

There may be some racial hate crimes mixed in there too, but if you look at the hate crimes report in Los Angeles vs. the overall murder rate, it's not really enough to budge the numbers much.