Studies have shown that students have been more willing to take this rice purity test than to take a very private STD test because they felt the purity test results were more of a no judgement zone.
Russians come from a plethora of different people, but Slavs are the most common. Slavs traditionally have light brown or even blond hair, with only a smaller part of them endowed with dark hair from southern ethnicities, like so: Black hair color is actually pretty common, especially the further you go East.
Brown hair, in particular, tend to be the most widely used color for ordinary Japanese characters, including the main characters, because a black haired female can indicate other qualities. Black hair (or dark blue and purple) can be more common among male protagonists, to make them more relatable to the audience.