The Minnesota Transracial Adoption Study looked at black kids who had been adopted, as infants, by middle-class to upper-middle-class white families. The purpose was to study the effects of environment on IQ. The study found that, even being raised from infancy by affluent white families, and going to school in good schools, the average adult IQ of the adopted black kids was just 89.
I have not heard of any further studies done.