I was looking into The Atlantic's past coverage of race and IQ issues, and found this 2001 article from Steve Olson that's really focused on other issues, but contained this paragraph:

Take IQ tests as an example. In Japan the Buraku are a caste of people discriminated against in education, housing, and employment. Their children typically score ten to fifteen points below other Japanese children on IQ tests—about the average black-white difference in the United States. Yet when the Buraku emigrate to the United States, the IQ gap between them and other Japanese vanishes.

This particular factoid seems to have come into the American consciousness through John Ogbu's Black American Students in an Affluent Suburb where he says:

However, there is a problem with their argument when it is placed into cross-cultural perspective: Differences in group IQ are not necessarily the result of differences in genetic endowment. Consider the case of the Buraku minority in Japan, which has lower IQ test scores and lower school performance than the dominant Ippan group in the Japan, even though the two groups are of the same "race." On the other hand, as immigrants to the United States, the Buraku minority and the Ippan majority do equally well on standardized tests, and both make good grades in school (DeVos, 1973; Ito, 1968; Y. Nabeshima, personal communication, August 30, 1999; Ogbu, 2001; Ogbu & Stern, 2001; Shimagara, 1991).

In a 1995 article, Nicholas Kristof wrote about the plight of the Buraku, concluding with emphasis on the enormous progress that has been made in recent decades, thanks to concerted effort to reduce discrimination and inferior living conditions:

Truancy rates in elementary school in 1960 were 12 times as high for buraku children as for others. Now they are twice as high.

Burakumin have almost caught up with their peers in the proportion who graduate from high school, a tremendous achievement. But only about 24 percent of burakumin go to college, compared with 40 percent of other Japanese.

Here's a 1973 Time article looking at rising demands for recognition and equal treatment. It's a fascinating subject -- Japan is usually portrayed in the US as an almost totally homogenous society and what discrimination I'd seen reporting about
in the past had to do with Koreans.