BACKGROUND: Compared to other racial/ethnic groups, Black Americans show accelerated brain aging beginning in midlife and exhibit higher rates and earlier onset of dementia. While these patterns are often viewed as evidence that Black Americans are more vulnerable to Alzheimer's disease, another possibility is that their high risk for chronic vascular pathologies such as high blood pressure and diabetes compromises their brain health. This hypothesis was tested in the present study. METHODS: Participants included 252 middle-aged Black Americans enrolled in the Family and Community Health Study and living in Iowa or Georgia who had ascertainment of diabetes and hypertension in 2008 and blood drawn in 2008 and 2019. Linear regression models assessed whether hypertension and diabetes were associated with change in neurofilament light chain (NfL), a non-specific biomarker of brain pathology including vascular dementia, and phosphorylated tau181 (p-Tau181), a biomarker of underlying Alzheimer's disease (AD) pathology. RESULTS: Having a diagnosis of either hypertension or diabetes during middle age was not associated with levels of either p-Tau181 or NfL. However, having a diagnosis of both hypertension and diabetes was robustly associated with higher levels of NfL and increases in NfL over an 11-year period. CONCLUSIONS: Among Black Americans, comorbid hypertension and diabetes contribute to higher levels of serum NfL, our indicator of neurodegeneration, during late middle age but not with p-Tau181, our indicator of Alzheimer's disease. Rather than being instances of AD, elevated rates of dementia among Black Americans may be largely vascular pathology caused by high rates of diabetes and high blood pressure.