IQ by itself isn’t a good predictor. Combine IQ with conscientiousness (from the Big 5) and we’re talking. Analogously, SAT and high school GPA.
Regarding blacks catching up, you can trust that Flynn knows that whites have advanced, and that blacks have been catching up to a moving target.
Perhaps you are right.
But that is not what I took away from the Bell Curve nor is it observationally obvious -- if you believe the IQ metrics in that book -- which I tend to believe.