Slaves were brought to the US from Africa by the English, French, Dutch, Portuguese, and Spanish. Most African slaves were sold to Europen slavers by their fellow slaves. A word of note, Obama's family in Africa has been definitely linked to the slave trade but not in a good way. I guess that means no statues of that former president. Anyway, back to the new world. The first African slave was brought to what would be the United States in 1637 by the English in the colony of Virginia. Have to remember that, English law, not US law. That wouldn't exist for another 150 years.
So we had three kinds of slavery in the new world; chattel, voluntary indenturement, and criminal indenturement. Occassionally England, France, and Spain would empty their prisons by sending the inmates (male and female) to the colonies. Criminals would serve out their semtence and be "freed" to fend for themselves in the new world. In many cases a voluntary indentured servant would make some wages so when their term expired they may have some money and a skill but that is entirely up to the owner. An owner could do what they wanted with their servant including beating and rape. It was all good, clean fun right? Here is the difference, an indentured servant was a human being and had some basic rights. An owner who killed a servant would have to answer to some justice. A slave owner could hang every single slave he (or she) owned and no one cared.
Back to that definition of a drop of blood; a grandchild of the original African slave could have been the result of two generations of forced sex by white owners. They could like white and if they escaped could probably pass for white but as long as they are in servitude were stuck being slaves. Thomas Jefferson's wife was the half sister of a slave named Sally Hemmings. The widower Jefferson took up with his slave Sally and had children with her. He took her to France where she served as the lady of the household. Why didn't he free her? He couldn't according to state law. Oh, he could free her but she would still be black according to the law and unable to marry Jefferson. She could not live with Jefferson without marriage in those days so their only option to remain together was as owner and slave. Jefferson was not the only. A future Vice-President of the United States Richard Johnson was married to a woman named Julia Chinn in Kentucky. She was considered to be 1/8th black and that made her black according to the law, like it or not. They had two daughters, both went to the best schools and both were married to men of property.
Other law makers also had odd relationships with women of color because of state law and contemporary beliefs.
Have to go, finish this later. If you have any questions...