They're failing pretty badly, if that's actually what they're trying for. A few hundred a year out of a population of 40-odd million is really not going to do it.

They might be doing something despicable, and many police departments act with criminal callousness towards black people (literally criminal "policing", c.f. Floyd).

But "kill all the people whose ancestors were slaves" isn't what's going on unless they're ridiculously incompetent at it.

Hard to find a solution if you don't correctly identify the nature of the problem.