Education worked the best in the 1960s: In my view, a rational education system would make sure that the correlation between IQ and highest degree earned is high. It's very simple: less intelligent people should end their educational career earlier, while smarter folks should pursue higher degrees.
Assuming that I'm correct, let's see which decade had the most rational approach for both blacks and whites. Using General Social Survey data, here is what I found:
Correlation between IQ and years of education--whites
This decade .36
Correlation between IQ and years of education--blacks
This decade .39
First, we see the highest numbers were in the 1950s and 1960s for whites. The system was most rational during that period for that population. It's gotten worse ever since.
For blacks, we see a small increase from the 50s to the 60s, perhaps indicating decreasing discrimination. After that, the numbers have fallen for blacks as well, and currently the correlation is low and similar to that of whites.
If I'm interpreting this correctly, we're seeing more and more blacks and whites stay in school beyond what their IQs would warrant.