You cite as an example the 2015 protests in the city of Baltimore. After these protests (and arguably as a result of them), the city of Baltimore and the Baltimore City Police Department (BPD) requested that the Department of Justice start an investigation into the BPD.
The findings of this investigation are publicly available and were released in mid-2016 [PDF]. It recognizes a pattern of racism and police brutality:
the Department of Justice concludes that there is reasonable cause to
believe that BPD engages in a pattern or practice of conduct that
violates the Constitution or federal law. BPD engages in a pattern or
(2) using enforcement strategies that produce severe and unjustified
disparities in the rates of stops, searches and arrests of African
(3) using excessive force
It also recognizes that this pattern is rooted in other systemic issues faced by the city, which themselves stem in large part from racist policies of the past:
Providing policing services in many parts of Baltimore is particularly
challenging, where officers regularly confront complex social problems
rooted in poverty, racial segregation and deficient educational,
employment and housing opportunities.
However, as you note, a people allowed to choose its elected officials should have better representation and therefore better policing. To that I would say that better policing means improving conditions, not instantly good conditions. The report continues on to express that there is a consensus that there is a problem and it needs to be fixed:
In the course of our investigation, we learned there is widespread
agreement that BPD needs reform. Almost everyone who spoke to us—from
current and former City leaders, BPD officers and command staff during
ride-alongs and interviews, community members throughout the many
neighborhoods of Baltimore, union representatives of all levels of
officers in BPD, advocacy groups, and civic and religious
leaders—agrees that BPD has significant problems that have undermined
its efforts to police constitutionally and effectively.
This investigation is one of the ways in which the issue of systemic racism and police brutality are being addressed in that city. The summary goes on to cite a few other ways in which current leadership is taking steps to tackle this systemic racism:
Fortunately, the current leadership of the City and the BPD already
have taken laudable steps to reverse this course, including by
revising BPD’s use of force policies, taking steps toward enhancing
accountability and transparency throughout the Department by, for
example, beginning to equip officers with body worn cameras, and
taking steps toward improving and expanding its community outreach to
better engage its officers with the community they serve. Still,
significant challenges remain.
So then, your question might now turn to why this was still a problem as recently as 2015? Bishop Robinson was Baltimore's first african american police commissioner in 1984. Baltimore has had democratic mayors since the 70's and had its first (elected) african-american mayor in 1987 with the election of Kurt Schmoke. The political representation is there and you should expect it to matter. For example Kurt Schmoke heavily criticized the War on Drugs which is recognized today as having disproportionately hurt african-americans.
Baltimore, historically, is home to heavily marginalized, impoverished african-american communities, as acknowledged repeatedly in the Department of Justice's report. These conditions leave these communities vulnerable to violence and in need of help, yet the police was used to contain and control rather than serve and protect. The 80s and 90s witnessed the crack epidemic. An incredible flow of cheap drugs into the inner cities of the United States, one which you should expect to hurt impoverished, hopeless communities the most. Combine that with the War on Drugs and you can begin to understand why the african american communities in Baltimore were in crisis. Political goodwill is not necessarily powerful enough to solve a complex, multilayered, interlocking set of difficult circumstances.
In addition, Kurt Schmoke's mayorship was followed by that of Martin O'Malley, a mayor who would push for a zero tolerance policing policy and heavy criticism of his predecessor's lax attitude on drugs, likely taking inspiration from other cities such as New York and their own zero tolerance policies. This notion that having a hyperactive police force which is tough on small crimes will naturally lead to a reduction in bigger crimes stems from the Broken windows theory, a popular new socioeconomic policy of the time.
This zero tolerance policy and focus on keeping numbers of arrests and stops high, the Department of Justice's report notes, had a very negative effect on the trust between the BPD and the african-american communities:
Throughout the 1990s and 2000s, arresting large numbers of people for
minor offenses was central to BPD’s enforcement paradigm; in 2005, BPD
made more than 108,000 arrests, most for nonviolent offenses.
These are issues that are known to the elected officials and that they want to resolve:
City officials also admit that the Department’s approach has been
problematic. Mayor Rawlings- Blake has long recognized the need for
reform and repeatedly criticized the aggressive policing strategies
championed in the years before her term.
Simply put, you are right to believe that a majority african american city should be electing officials who will better represent them and work to resolve issues of systemic police brutality and racism. The issues that are faced are ones that are deeply rooted, difficult to resolve, but that are being thought about very seriously and that are being worked on.
Sometimes, catastrophic events such as an unexpected flood of cheap drugs combined with mandatory minimum sentences force progress to take two steps back. Sometimes, an attempt to resolve issues by applying a new concept of policing such as zero tolerance policing causes more lasting harm than good.
I encourage everyone to read the Department of Justice's report, there is a lot that my answer doesn't cover, and it gives you an insight into the world beyond what you can get from quick answers, articles and 24 hour news cycles.