These numbers require more context to be meaningful. First, this was for his second term, which typically sees an increase in support. Second, Biden was instrumental in quite a few policies over the years that were generally bad for black Americans, and he was extremely critical of the BLM and police reform (de-fund) movements. Third, the percentage of black Americans which don't live in poverty has increased, and middle class black Americans are more likely to be fiscally conservative for the same reasons that all middle class Americans are more likely to be fiscal conservatives than those who live below the poverty line.