That quote seems to have originated from an article published on The Root in 2013 and written by the renowned scholar Henry Louis Gates. In the article Gates is quoting Stanley Engerman, an economic historian who has written on slavery as an economic system. Gates does not give the exact source for the quote (unless I missed it it somewhere). Engerman wrote a controversial and sometimes criticized economic history of American slavery, but given that it would be an easily verifiable statistic and Gates himself is quoting it, I don't think its an inaccurate statement.
Charleston was somewhat of a unique city in 1850. In 1850, there were few relatively large cities in the South, with New Orleans, Charleston and Richmond being the largest, and only New Orleans being in the top 10 in terms of population in the entire US. Charleston lay right in the middle of an area of intense plantation agriculture, meaning that there were many large plantations with large enslaved populations not far from the city and the city become the economic hub for that plantation rich region with many economic activities relating to slavery happening in the city.
In 1850, Charleston's population was 42,985. Of this, 3,441 were free Blacks and 19,532 were enslaved Blacks; meaning about 1,445 free Blacks owned slaves in Charleston in 1850. The full quote also says that 64% of these people were women.
Unfortunately I don't know much about slavery in Charleston during this time period to go into any depth about slavery there, but I would highly suggest reading the above-linked Gates article since it goes into detail about how the phrase "owned slaves" might be misunderstood when it came to Blacks owning slaves in the Antebellum South. Basically, many Blacks who "owned" slaves had purchased members of their families, and for whatever reasons (i.e. states often made it difficult to free slaves) never formally freed their "slave;" although there are cases of Blacks owning a large number of slaves for the purpose of economic exploitation.
By 1800, one third of colored heads of households in Charleston had slave property. Between 1820 and 1840, the percentage increased to 75 percent. By 1850, the percentage had declined to 42 percent. (Davis 1994).
Source Page 18
EDIT: Note that it says "heads of households". So considering that the freedman's family isn't being counted in the total freed population, the 42% figure is somewhat misleading.
Not much to add, but just wanted to confirm the quote. I couldn't find a pdf of Engerman's book online, but that article is great resource on the topic. It's also important to note that urban black-owned slaves were owned largely by women, and worked as house servants (often rented out to white households.)
If this is a true figure, or close to it, I have a question. I had been taught that most whites in slave owning areas did not own slaves, because they were expensive. Now, 42% owning is still most not owning, but it surprises me that such a large percent of free blacks in the slavery era south would advance enough economically to be well off enough to own slaves. How were they able to accomplish this?
ADDED: What percentage of whites owned slaves?