"Mixed Race" births were not so common 50 years ago, and I'd wager in Hawaii, black/white mixed race births even less so.
Hawaii has an interesting perspective on race, and always has. My son is dating a Hawaiian girl, and I have met her family several times. They explain that race has never been important to Hawaiians. A majority of the population in Hawaii is mixed race.
Kai lai’s parents laugh about Obama saying that growing up black in Hawaii was difficult. They say that is all made up to get sympathy for him. According to them, even in the 1960s-1970s, most Hawaiians would not even have noticed that BO was mixed race, let alone cared. They say it would have been extremely odd for anyone to have even mentioned the race of someone, for any reason. Apparently it was a true melting pot of many different races that all intermixed freely.