A small genetic component but the larger factor is attributed to (lack of) early childhood nutrition in predominently urban black communities.
American slavery was about warehousing labor for Cotton Harvest (and cotton planting, to a lesser degree), and doing so at the cheapest per-unit cost. This is why "Soul Food" uses so much cast off meat cuts and other things of generally decent caloric quantity but poor nutrition. Basically blacks tend to grow up with a limited, mainly processed, diet that leaves children malnourished on key trace nutrients. Early child hood nutrition is not something you can really get back, so even if you improve your lot in life, you will always be a little smooth-brained than peers. White, even poor whites, tend to prioritize their kids and making sure their kids have whatever the doctors are saying kids need this week. The extended family structures are usually different (or I should say are currently and traditionally different) where there are grandparents instead of just a network of loosely affliated cousins and aunts, sometimes with grandma in there somewhere - in so much as the grandparents will ensure the kids are fed well on at least a semi-regular basis. Rural poor whites also have "hunting" available, something innercity blacks lack access to.
The convergence died down in the 70s as social service policies heavily disincentivized marriage and favored single motherhood, destroying what had been incredible process by blacks towards normalizing to white statistics (when corrected for socioeconomic factors)
Humans are very flexible and adaptable. While there are definite predilections, most anything 'genetic' can usually be accounted or corrected for at a social/cultural level and not a genetic one. (For example, men with Double Y have extra testosterone, and are over represented in the prison population because it tends to make them violent. But not everyone with Double Y ends up in prison. Just more than average.)
Sorry. I meant to post "Hang all niggers, gas the kikes" but typoed.