I like how investopedia explained it:
The Black tax, a term that originated in South Africa, is the financial responsibility placed on family members who experience upward mobility to help out their relatives who are less fortunate. Often, the successful person is the first one in a poor family who graduates from college or attains a high-paying job.
My parent sent all of us in my family to school, up to university level. And my father always said if anyone is ready for MSc and PHD, that he is ready to pay for it.
The money that I give them is not a black tax for me and they are worth it. Not that the depend on it.
For a poor family that sent their first born to school, I expect the person to help his siblings. But the state Nigeria has been for a long time, unemployment and underemployment has increased which can make this not to be possible in Nigeria because a graduate that is not able to cater for himself and his wife, will not be able or find it hard to help his family members like siblings and other relatives.
About siblings, it is better parents help them in a way one will not depend on the other. At least, they should all go to school which may help even if they are unemployed or underemployed.