[DrBrightonone]

IMO, it was originally for economic reasons. It was a way to have a more sizeable slave population. Slaves were very valuable. The more you could produce, and the more prosperous you were considered to be. After emancipation, to keep "blacks" and poor whites from aligning against the planter class, it later evolved into racial hierarchy and segregation that was legally mandated.

By the 50s, it was flipped around more positively when Afro-descended people from white looking to blue-black came together to form a political power base to get segregationist laws and policies off the books. This lasted through the early 70s, when black was beautiful. I grew up in this era.

However, now, with Afro-descended people of various nationalities, the cohesiveness that went along with being "black," which was empowering, has been diluted.

ODR it is not "the law" in the US at the present time. The US Census asks people to self-identify. I expect to see some changes in how people self-identify as there are more Afro-descended people who are from places like Latin America and the Caribbean, who are racially as well as culturally diverse, more US born who are bi- and multiracial, and those (like me) who always self-identified as black (although we were fully aware of mixture in our ancestral pool), because the other parts of us were never acknowledged. I became aware of my full admixture from recent DNA testing.

Personally, I will probably continue to self-identify as "black" until I can figure out a term that best describes what I am. I never did like the term "African American." It is too vague.

This should explain it from a historical perspective http://en.wikipedia.org/wiki/Hypodescent