Diabetes scales with obesity and age. Japan has a very old population, but less obesity than the US. White rice despite its glycemic index is not super high calorie. If it's a contributing favor, it's because of weight-gain, not glycemic index.
There are also other food items which have versions that are black in colour, some of which could be not naturally, but by processing them in some way, for example, black garlic.
It’s the Japanese equivalent of “cup” as a measurement unit, and at 180 ml is about 76% of a US cup. Japanese rice cookers are marked and filled in units of gō. You put n gō of uncooked rice in it, and then fill water up to the n mark. One gō is roughly 150 grams of uncooked rice, which amounts to around 530 kcal. Traditionally, a wooden box was used to measure one gō: https://commons.wikimedia.org/wiki/File:Masu,_One-Gō_measure...
A Japanese measuring unit for rice, kind of analogous to "one cup" in US kitchens. Measuring cups distributed with rice cookers are usually one gō in volume even in the US.
Japan has a 12% diabetes rate, which is just above the US at 11%.