The highest blood lead levels are seen in children living in denser population areas at or below poverty thresholds. Homes in these largely urban, low-income areas are disproportionately built before 1978 and subject inhabiting children to a significantly higher lead exposure risk. In 2014, it was estimated that 90 percent of all DC homes were built before 1978.
Despite Baltimore leading the charge to inform the public concerning the risks of environmental exposure to lead by banning lead-based paint (but not plumbing or other construction products) in 1951 following a clinical study from Johns Hopkins University, many Baltimoreans, primarily Black, continue to face excessive exposure to lead. The US would not follow Baltimore's example and ban lead-based paint until 1978, because lead lobbyists continued to disrupt regulation efforts.
Lead remediation is an expensive process and even with programs that help or completely cover the costs, many people may not be aware of the programs to take advantage of them.
You can start to see why the problem disproportionately impacts the Black population.