Mdthbs

This is not quite a duplicate of Did American slave holders typically give their slaves the names of Roman nobility?. Over there, two sheds' answer gives a compelling argument that it wasn't so much that slaves had classical names, as everyone had classical names in the 19th century, from Ulysses S. Grant to Junius Brutus Booth. The argument is that if you saw the name "Cornelius" on a résumé in 1830, it might just as well be a white American's name as a black American's. I have no reason to disbelieve this argument.

However, if I saw the name "Cornelius" on an American résumé in, say, 1940 — or 1970 — or 2010 — then, frankly, I'd guess that the holder of that name was black.
The same goes for most of the other classical names on two sheds' list: Julius, Lucius, Octavia, Brutus, Cornelius, Cassius, Marcus, Rufus, Cyrus, Darius.

This isn't to say that there aren't any white kids named Marcus. But my impression is that these names have had more staying power with African-Americans. So my question is, "Why?"

Part of the answer might be conservatism-in-naming; that is, you name your kid after his grandfather for a few generations, and suddenly you have a bunch of Luciuses running around in 2019. Conservatism among blacks (black Americans are more religious than white Americans, for example), and/or conservatism in the Deep South (example) followed by exportation of the culture elsewhere.

Part of the answer might be that certain names become "traditional" by being names of heroes — I don't think of Martin or Rosa as "black" names, but I bet they're statistically more frequent among black families these days — and perhaps the 19th century (when a lot of people had classical names) was a fertile time for heroes in black culture more than in mainstream culture? That could explain a bump in popularity for Marcus and Cassius at least. (And of course Cassius got a second bump in the '60s.) But that hypothesis doesn't explain all the classical names, just a few of them.

Could someone with more historical/cultural knowledge try to paint a fuller picture here?

Classical names were given by masters to slaves from the very beginning of the slave trade.

Classical names abound in eighteenth-century slave records. Even though slaves were not, as a rule, given new namess until sold to their new owners, the few instances of slave traders or sailors naming their cargo before they even reached America indicate that these chained Africans were dubbed Caesar, Nero, or Pluto, or, as the first two names on one list of cargo, Primus and Secundus.

Inscoe, John C. “Carolina Slave Names: An Index to Acculturation.” The Journal of Southern History, vol. 49, no. 4, 1983.

These names originated from a peculiar kind of Southern wit. It is well-known among Southerners even today that the names of classical heroes were given to slaves as an ironic insult to their status as slaves. It also had the purpose of demonstrating to visitors the plantation owner's own learning.

Southerners prided themselves on their knowledge and appreciation of Græco-Roman civilization and often stressed the many similarities between it and their own society, not the least of which was the institution of slavery.

Inscoe, ibid.

Occasionally a name insulted the specific personality of the slave, but more often it created a vague Classical aura which gradually lost its degrading tone.

All but one of eight slaves named in William Yeates's 1751 will were named for Roman gods or heroes. The names of almost a third of South Carolina runaway slaves [...] were of Greek or Roman origin [...] there seems to have been little correlation between the original ancient figure and his black namesake, except for an occasional dim-witted male slave named Plato or Socrates or a sexually promiscuous girl named Venus or Aphrodite. Thus, the intent of the owner in bestowing these names would have been too subtle or tenuous for the slave to have detected anything insulting about them. Nor can one assume that all of these names were first applied by masters for satiric or condescending reasons, particularly once the practice became commonplace.

Inscoe, ibid.

The practice of having masters name slaves was widespread throughout the South.

Slave children, in fact, were often, if not usually, actually named by the master or mistress. [..] Classical names, although less numerous than certain writers on plantation life would have us think, also probably reveal the hand of
the master class. Our slave list includes the following: Achilles,
Augustus, Bachus, Brutus, Calypso, Cassius, Cicero [...] Scylla, Silla, Siller, Sylla.

Puckett, "Names of American Negro Slaves," in Murdock, ed., Studies in the Science of Society (New Haven, 1937), pp. 471-494.

This 1937 source claims elsewhere that classical names were also common in the white population, but this is not born out by census data.

Like place-names and day-names, classical names were rarely found among the slaveholders. These names were associated almost exclusively with slaves. Prior to 1800, classical names accounted for about 20 percent of names given to male slaves born on the Ball plantation. During the nineteenth century, the share of classical names declined to about 10 percent.

Cody, Cheryll Ann. “There Was No ‘Absalom’ on the Ball Plantations: Slave-Naming Practices in the South Carolina Low Country, 1720-1865.” The American Historical Review, vol. 92, no. 3, 1987, pp. 563–596.

Ulysses S. Grant (né Hiram Ulysses Grant) was born in Ohio to an originally Puritan family who probably had different ideas about naming; the 1937 source observes that the prominence of the name Ulysses probably derives from him, not from antebellum names.

20th century historians remark on the classical interests of slave owners, but 21st century historians have been unable to avoid noticing the inherent cruelty in assigning someone a name that serves as a source of fun. They also observe that the names tend to play on skin color, either through reference to ancient Africans, or through juxtaposition.

"Such names functioned as cruel jokes: for instance, Scipio, a common male slave name, referred to the Roman general Publius Cornelius Scipio Africanus, whose agnomen, Africanus, meant “the African,” in praise of his triumphs in battle in North Africa." (Abel, Tyson, and Palsson, 2019. "From Enslavement to Emancipation: Naming Practices in the Danish West Indies." Comparative Studies in Society and History, 61(2), pp.332-365.)

"Such names served to reinforce the idea of Africans as embodiments of exotic alterity, but also invited pointed comparison between the appearance and circumstances of the slave and the illustrious personage referenced by his name. These were names to call as a joke, names whose grandiosity humiliated: Ignatius Sancho, Gustavus Vassa,
Julius Soubise." (Susan Benson, "Injurious names: naming, disavowal, and recuperation in contexts of slavery and emancipation." Vom Bruck and Barbara Bodenhorn, eds. The anthropology of names and naming. Cambridge: Cambridge University Press, 2006.)

"[The literate slave trader] chose not to speak of Venus, the other dead girl. The pet name licensed debauchery and made it sound agreeable." (Saidiya Hartman, Lose your mother: A journey along the Atlantic slave route. Macmillan, 2008.)