It's complex, and defies simplistic explanations (so don't believe simplistic explanations if you see/hear them).
An attempt at a nuanced explanation (which undoubtedly misses a ton of factors):
Huge disparities in overall wealth and income, coupled with geographic segregation and isolation of a lot of the black population in areas of concentrated poverty, and the associated lack of access to social goods, such as public safety and good schools. The combined effects of poverty/segregation/isolation are specifically reflected in the fact that 78% of homicides involve people who know each other [1]. People who know each other killing each other is a result of the breakdown of community structures that would disincentivize that behavior. There have to be powerful incentives for someone to kill someone that they know, considering that the potential consequences are so severe (incarceration, violent retribution, etc). Often the perpetrators have very little (in wealth, community, etc) to lose, which also increases the incentive.
Another group that are similarly economically poor, Latinos, but due to a different history, have more intact community structures, don't have as high a rate of violent crime.
[1] https://www.bjs.gov/content/pub/pdf/htus8008.pdf (Table 8)