If you are having trouble finding sources, it’s likely due to the terminology you’re using. You don’t usually hear the terms “comfort girls” or “comfort women” in the context of America slavery. A more common term is “fancy girls,” or “yellow girls,” referring to enslaved women of mixed race.

Black women, especially light-skinned Black women, were definitely fetishized and commodified as sex slaves in Antebellum America. Depending on the wealth and temperament of their owners, these women were sometimes treated relatively well. Sharony Green, citing J. Winston Coleman, writes that Lewis Robard’s “fancies were housed in nicely furnished apartments above his Lexington office” and “they engaged in needlework while waiting for a buyer” (21). Like white sex workers, though, their living conditions ran the gamut between “high class” situations like with Robard; working class, hired-out prostitution; treatment as otherwise “regular” house servants; and complete and utter depravity and abuse. All, of course, within the parameters of slavery, where they had little bodily autonomy and often could not decide who they slept with or keep the funds they earned with their own bodies.

Rarely - but it did happen - white masters would free their favorite enslaved women, as in the case of Avenia White and Rice Ballard (detailed in Sharony Green’s article listed below). In Avenia’s case, Ballard freed her and continued supporting her for a time before eventually abandoning her completely.

One of the most famous instances of a relationship between a white man and his enslaved mistress was Thomas Jefferson and Sally Hemmings. Sally was mixed race and had several children by Jefferson. By all accounts, however, Jefferson and Hemmings’ relationship was outside of the norm - Jefferson was elite, and Hemmings was (allegedly) a more willing participant than many other enslaved mistresses would have been. Edit: this is a problematic statement and I shouldn't have made it without going into more detail about the nuances of their relationship. See comments below.

Sources:

“‘Mr Ballard, I am compelled to write again’: Beyond Bedrooms and Brothels, a Fancy Girl Speaks” by Sharony Green - includes a very useful historiography

“The Fancy Trade and the Commodifification of Rape in the Sexual Economy of 19th Century U.S. Slavery” by Tiye A. Gordon

“‘Cuffy,’ ‘Fancy Maids,’ and ‘One-Eyed Men’: Rape, Commodification, and the Domestic Slave Trade in the United States” by Edward Baptist

“Thomas Jefferson and Sally Hemings,” a monograph by Annette Gordon-Reed

A good search term to use for this is "chattel slavery"

"The slaveholder has it in his power, to violate the chastity of his slaves. And not a few are beastly enough to exercise such power. Hence it happens that, in some families, it is difficult to distinguish the free children from the slaves. It is sometimes the case, that the largest part of the master's own children are born, not of his wife, but of the wives and daughters of his slaves, whom he has basely prostituted as well as enslaved. "

Rankin, John (1833). Letters on American slavery, addressed to Mr. Thomas Rankin, merchant at Middlebrook, Augusta County, Va. Boston: Garrison and Knapp.

Slaves were property primarily for labor and/or for sex pretty much universally. For the slave owning classes, sex was often socially proscribed amongst themselves, but what you do with your "property" was your business, so slaves provided a sexual outlet:

Davis, Adrienne (2002). ""Don't Let Nobody Bother Yo' Prinicple" The Sexual Economy of American Slavery". Sister Circle: Black Women and Work. Rutgers University Press.

Slavery is also not a "binary thing", there are degrees. Look up "The King's Daughters" where King Louis XIV "needed" to build the population in his "New France" colony and so arranged for shipping white "wives" to the men in these territories. This was sort of "free" but often defacto forced by poverty or prostitution convictions (often again from poverty) and the marriages nominally free, but in fact one had limited time and stark choices to find support in fairly unsettled and dangerous lands overseas. Let's entertain that as a 50% "free" choice, vs an African slave 0%?

Gagné, Peter J. (2002). Before the King's Daughters The Filles à Marier, 1634-1662. Quintin Publications.

Finally, there is the most famous case of Thomas Jefferson and Sally Hemings. She negotiated that her children would be set free when they turned 21, so she had a bit of influence even a slave which could imply a bit of a two-way relationship. Even in this most famous case, nothing is 100%, it's not like the slave owners had any incentive to document their sexual relationships with their slaves and the slaves were mostly incapable of documenting anything due to remaining slaves, uneducated, or both -- so real historical sources will be scarce on your topic. I do in a way absolutely love the case of Jefferson because I personally regard him as one of humanity's greatest statesmen while at the same time he owned slaves, which he knew was both morally and for the USA structurally disastrous and yet he couldn't/wouldn't personally separate himself due to his own economics nor publically due to political realities of creating the United States rather than the un-united states. This is why he encapsulates truths and contradictions of being human and of humanity so well for me. His best principles became universalized and are foundational still to freedom in the world, his failures stain his grave. IMHO, we need to deal with both.

"Monticello Affirms Thomas Jefferson Fathered Children with Sally Hemings". Monticello.org. Thomas Jefferson Foundation. 2018. Retrieved August 12, 2019.

I would also suggest looking into the 19th century trafficking of Asian women during the San Francisco gold rush. Because there were no established towns, the male to female ratio was considerably skewed. Chinese immigrants often ran service related sectors that catered to the miners and prospecting parties, and they established a lucrative system of trafficking in Asian girls and women for sexual slavery and indentured servitude.

American prostitutes and brothels were present, and certainly many were trapped into prostitution, but there was also a contingent of business savvy women who already worked in the sex trade who realized that there was a high demand for their services and so put up stakes. While I am in no way lessening their plight and circumstances, the standards they lived in were much better by far than those of the Asian sex workers, and their prices more inflated, which created an economic divide that made the Asian sex slaves more likely to be frequented by the less financially stable men.

The Asian sex workers lacked the resources and freedom to seek treatment for std's, the health problems rampant due to the poor living conditions, and (albeit limited) autonomy American prostitutes had, and thus no recourse to limit their exposure to more dangerous and abusive customers.

Unfortunately, there are few first hand accounts; most of the girls brought over came from impoverished families and lacked any formal education that would have given them the ability to write and had language barriers that precluded their ability to even tell their stories to the American abolitionists who would listen. More importantly, cultural traditions imposed an unbearable burden of shame upon the few women who did survive and make it out; there are stories of suicide and forced segregation from society among some who returned to their home countries. Even now in modern times, when San Francisco installed a memorial statue to honour these comfort women, there was immense pushback from Japan.

A few resources:

• The White Devil's Daughters: The Fight Against Slavery in San Francisco's Chinatown by Julia Flynn Siler

• Contagious Divides: Epidemics and Race in San Francisco’s Chinatown by Nayan Shah

• Relations of Rescue: The Search for Female Moral Authority in the American West, 1874–1939

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An article about Tye Leung Schulze, a Chinese American suffragette who not only helped to rescue sex slaves from Chinatown, but was also the first Chinese American woman to vote, the first to pass the civil service exams, and the first to have a government position. Her life is absolutely fascinating.

• https://wednesdayswomen.com/tye-leung-schulze-womens-rights-advocate-first-u-s-chinese-voter/

And this is a book that focuses on both the topic and her:

• Unbound Feet: A Social History of Chinese Women in San Francisco by Judy Yung

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Donaldina Cameron was a Presbyterian missionary who lived in San Francisco and worked to rescue thousands of Chinese women and girls forced into sexual slavery. There are a couple books about her efforts, but some of the most recent ones are academically critical of the underlying issue of a white woman in the role of saviour to these women. A large part of the issue is that when many of the women were first rescued, she in turn did everything possible to convert these women into their religious beliefs, and arrange marriages for them.

It's a double sided blade- certainly her intentions were not malicious, and the women she rescued lived in substantially better conditions than they did as sex slaves, but they still had little to no autonomy. Her work is a valuable resource for information, but it's important to look at the situation through a modern lens as well as a historiographic one.

• Fierce Compassion: The Life of Abolitionist Donaldina Cameron by Kristin Wong, Kathryn Wong ·

• Donaldina Cameron: A Reappraisal by Laurene Wu McClain

• The Donaldina Cameron Myth and the Rescue of America, 1910–2012 by Kirsten Twelbeck