Just for some background, this isn't really saying Black Alzheimer's is more less genetic than White Alzheimer's---we don't know yet.
The reason the research was done is that a genetic variant in White populations that increases Alzheimer's risk, a variant of a locus on the APOE gene, does not raise the Alzheimer's risk in a similar fashion in Black populations. If this research is true, this is basically just the APOE for Black folks.