Mr. O. grew up (partly) in Hawai'i. There, to qualify for some nativist assistance programs, you have to document 25-50% Hawai-ian-ness. To qualify for a net-fishing license (everybody else has to use a pole and line) you must show 1/16th Hawai'ian-ness. So, there is some precedent to the 1/16th "rule" (I think I've read similar requirements for Navajo, Apache and other SW US natives for qualifying for programs).

Anyway, Obama picking "Black" reflects his personal identification since he left the islands, I guess, even though none of his ancestors were ever slaves. According to family legend I'd be 1/32nd Chippewa (a great grandma married to a French-Canadian ancestor), but I didn't check "Native American" on the census - not cuz I'm ashamed of great-grannie (to the contrary) but I know I don't much look Chippewa, more Irish-French. I really wanted to scrawl "American!!" in black crayon on the form, but figgered I'd get a vist from a Fed if I did. Then I'd hafta yell at him, kick him off the property, then the black choppers would come, and...