Most experts would agree that race, as both a social and scientific construct, developed in the eighteenth-century during the Enlightenment. If you wish to explain the development of the idea of race and how it led to racism, I would strongly urge you to start with certain Enlightenment thinkers, particularly Immanuel Kant and David Hume, who believed that phenotypical differences in race (i.e., the traits of dark skin, curly hair, wider noses, and thicker lips) were signs of mental inferiority. In the United States, Thomas Jefferson usurped some of these ideas when writing "Notes from Virginia." In this tract, he claimed that, while the indigenous people of North America could be assimilated to accept Western values, Africans were inherently incapable of being civilized.
In the 1690s, race became the primary indicator of who would be considered a free person in the American colonies. Race meant that free black people could not testify against whites in court. Race meant that enslaved blacks could not own property or even reap the rewards of their own labor. I mention this last point because some enslaved blacks had special skills, such as carpentry or blacksmithing. Their masters would sell their services, usually in towns, and reap the profits.
For black women, specifically, race meant the children they bore did not belong to them. The status of the child—that is, whether he or she was born free or a slave—was determined by the mother. This is why the offspring of slave owners were also very frequently considered enslaved persons. Black women could not choose their sexual partners or husbands freely. Though there were wedding ceremonies on plantations (i.e., jumping the broom) they were not recognized by law. Therefore, a black woman who considered herself to be married, could still be forced to have sex with a white man against her will. She was most likely to be raped or taken into concubinage by a slave master or an overseer—someone on her plantation who exacted authority over her.
As Sojourner Truth pointed out in her famous speech delivered at the Women's Convention in Akron, Ohio in 1851, race excluded black women from notions of femininity that both protected and subordinated white women. Black women, very often, were expected to perform arduous field labor, just like black men. However, they were also subjected to sexual abuse and were denied rights over their own children, who were often sold away from them like livestock. More fortunate black female slaves worked in the plantation house as domestics. However, they were not only given the responsibilities of cooking and cleaning, but also serving as wet nurses and nannies to white children.
Thus, race subjected black women to all of the vulnerabilities that have confronted women throughout history, but their race denied them all of the deference shown to white women. It is important, too, that white slave mistresses did not usually identify with or sympathize with black women, and they often tolerated their husbands', fathers', and brothers' sexual violation of black women.