It's not about skin color but culture and familiarity. Many of these guys grew up together, knew each others families, and could vouch for each other. It made LCN strongvand kept rats out for decades. Many still refuse to rat because of friendships and family. For every rat there are 25 that don't rat.
And out of the 25 that don't rat, 10 haven't being tested.
It's easy not to rat when you haven't served any more than 2 or 3 consecutive years in prison at a time, or when you don't have anything to offer the government.