In black Americans, the percentage of those with partial D and true Rh negatives may be around the same
The reason:
90% of Africans categorised as Rh- are actually partial D.
Since European percentages of Rh- blood are significantly higher, the percentage difference in ancestry evens it out.
Of course, it depends on the African tribes responsible for ancestries.
The claim that around 5 percent of black Americans are Rh- could be true.