There is discrimination against black people who are born in the US. This discrimination is linked to poorer health outcomes and poorer healthcare outcomes.
Someone lives in Nigeria and moves to the US as a 30 year old adult. They're black, but they don't experience that same sort of link between discrimination against blacks and poorer health/healthcare outcomes.
They then get married and have a child in the US. That child is still a member of the Nigerian ex-pat community, but because the child is born in the US, their experience growing up is like that of any other black person born in the US. Due to better social integration in the US, they experience the racial discrimination, and have health and healthcare outcomes, that are more similar to a US native than to the other Nigerian immigrants (who grew up in Nigeria).
So due to social integration in the US, children of immigrants experience a link between discrimination and poor health outcomes the same way that native US people do, and this is very different from their parents who aren't as socially integrated in the US.
I feel like I"m talking in circles a little bit here, but does that help clarify it?