For the past 8 years or so, the U.S. Justice Department has been led by two African Americans.
It is a federal crime to deprive someone of their constitutional rights (including the right to life and personal safety) "under color of law" (i.e., while acting as a government representative).\
Therefore, if the U.S. Justice Department concludes that any citizen - "Black" or otherwise - has been harmed wrongfully by police, then they are obliged to bring a case under Article 42 of the United States Code.
The fact that there are so few of these cases, and in particular in the cases that gain so much public notoriety (e.g., Michael Brown), is a pretty clear indication that the police are not "murdering" "Black" people in any significant numbers. Indeed, the number of "Black" people killed by police is about half the number of "white" people killed by police every year, which means that - with reference to documented criminality - a "Black" person is significantly less likely to die at the hands of the police than a "white" person.
But carry on. Don't let the facts get in the way of your self-satisfying victimistic narrative.