http://www.dinsdoc.com/goodell-1-1-23.htm
check the above for an interesting essay about the legality of slavery in the United States.
The conclusion of the author is that slavery never actually existed as a foundation in law- and this makes sense, ultimately there is no "general enslavement" decreed by law, rather it was just taken for granted that some people were slaves. It was a hybrid, artificial monster: slaves were both chattels yet also somehow held legally responsible as persons.
Also explains a lot about why "colored" slavery- it was easier to make this assumption on someone who is identifiable by birth (even though there were many free persons of color)
The whole issue was one of jurisdiction...which is especially moving when one realizes that a people who wake up to their own heritage and Gods Will stop being slaves at that instant.
That's why so much effort, then as now, goes into denying knowledge and truth. We are headed back to this condition of slavery, by our own ignorance.
The slaves of back then may not have realized that they were free...they would often agree that they were the property of a master and subjects of the slave laws. Probably those who wanted their freedom and were more self-aware were more likely to get or stay free overall. Its really about character and spirit.
Looks like we need another Moses...