An Elephind newspaper database search reports that an early instance of blackbirding used in this slang sense appears in an unidentified item in the Philadelphia [Pennsylvania] Press (April 17, 1861) [combined snippets]:
The bark Sarah, after having been towed down the lower bay, New York, on Monday afternoon was overtaken by the steam tug Only Son, and attached by Officers Thompson and Donnell, and brought back to the city and anchored under the _enns[?] of the United States steamer Vixen off the Battery. She is a small vessel, of 260 tons, and was built in Scituate [Massachusetts] in 1854. Her clearance, if she had any, does not appear in the papers of to-day. Her stern is painted black, with the name Sarah only on it. The general impression is she was bound "blackbirding."
Unfortunately, this particular newspaper comes from the Pennsylvania State University newspaper archives site, which has been offline for several months, so the photo image of the article is not available.
However, an earlier instance appears in an item from an issue of the [New York] Emancipator (November 1836), reprinted in The First Annual Report of the New York Committee of Vigilance for the Year 1837 (1837):
It is also pretty well ascertained that there is, at this moment, a vessel in this port [of New York], from the South, after a cargo of human beings! One of the hands, named JOHN PIERSON, was heard to say to a friend of his, whom he happened to meet, and who inquired what he was here for, "O, we've come blackbirding again."
Let parents, and guardians, and children take warning. Our city is infested with a gang of kidnappers.—Let every man look to his safety. Look out for the CUSTICES, for WADDY, BARNES, OWEN, RUTHERFORD, and young WOLFOLK!
Three years later, we have this instance, from Henry Milton, Rivalry, volume 3 (1840):
Holcroft, as the arrangements proceeded became so pleased with his occupation—it so strongly reminded him of some of his kind actions on the coast of Africa—that he grew quite chatty and good tempered.
"I have often before now," he said, addressing his silent companion, "laid traps to catch blackbirds without wings; but I never schemed such a pretty contrivance as this to let two knaves have a chance of getting off. Curse me if they would be worth the trouble, if it wasn't for the fun of the thing!"
In the same year, George Hill, Hill's Yankee Story Book and Negro Lectures (1840), a burlesque treatment of various topics rendered in pseudo-Black dialect, devotes five pages to "Deacon Gumbo's Lecture on Black-bird Pie," which notes that contemporaneous sailors often spoke of going to the coast of Africa "to get blackbirds," meaning to capture and enslave people living on the African coast. Hill plays the situation—and Deacon Gumbo's lecture—for laughs, which is sufficiently repellent that I won't quote the content of the lecture at length here.
From William Hurton, Oriana; or, The Wreck of the Arctic Regions, serialized in True Briton (November 4, 1852):
"And so you shall [be added to the crew of the 'Lady Emilia'], my man!" exclaimed I, "for we shall not sail short-handed. What name d'ye hail by?"
"Blackbird Jim, sir!" replied the man, who was a deeply-bronzed and thorough-looking seaman. The soubriquet of "Blackbird Jim," as I subsequently learnt, had been bestowed on him because he had for many years been engaged on the African coast in the respectable and lucrative profession of "blackbird-catching" i.e., aboard a slaver, and under this name he had been repeatedly entered on ships' books, for sailors frequently sign articles and sail under the most extraordinary nicknames conceivable.
And from John Hotten, The Slang Dictionary, Or, The Vulgar Words, Street Phrases, and "Fast Expressions of High and Low Society (1864):
BLACKBIRD-CATCHING, sea Slang for the slave-trade.
Another early instance of possible interest occurs in "Seizure of a Supposed Slaver," in the Richmond [Virginia] Enquirer (April 28, 1857), reprinted from the New York Times (April 25, 1857). The article describes at some length the circumstances underpinning the seizure of a schooner called Merchant, after a U.S. marshal, posing as an ignorant "countryman" from upstate New York in search of a brother who worked aboard an unidentified steamboat in New York Harbor, gathered information on the suspected slaver. Here is how the article describes part of the conversation between the disguised marshal and workers aboard a nearby vessel (not the Merchant):
The countryman [Marshall Rylands] was mightily amazed at the sight of so many ships, and was curious to know where they were all going to. Thy told him some wonderful stories about many of the ships, the height of waves during a storm, and all about whales and porpoises, and sharks. At lengththey came to the object of the countryman's affections. They pointed out the little schooner [the Merchant], and told him she was a "Blackbird." The countryman wanted to know if they caught blackbirds in her. They laughed again. The colored fellow [the ship's cook] put down his saucepan and roared again. They asked him if he didn't know what a blackbird was. He thought he did. He had caught more than one. But what do you mean calling that little vessel a blackbird? asked the countryman.
The stevedore that the marshal is talking to then explains that the boat will soon be going to Africa on a slaving mission, that it has room to hold 360 captives, plus "rice and such things" to feed them with on the return voyage, plus "lots of shackles on board."
It thus appears that sailors and dock workers in New York City in 1857 were familiar with blackbird as a slang term meaning "ship used to transport slaves."
Much earlier and probably on point is this instance from an untitled item in the Alexandria [Virginia] Gazette & Daily Advertiser (February 23, 1821), where black-birds seems to refer to "captive Africans bound for slavery":
We noticed a few days since, on the authority of our late Havana [Cuba] papers, the recapture of a Spanish Guineaman, from the prize crew of a Patriot privateer, and her arrival at Havana. The following extract of a letter, with which we have been politely furnished, gives some further particulars—
Havana, Feb. 5.—Our trade between this and Matanzas [also in Cuba], has been, and I may say now is annoyed by Patriot cruizers—a capture was made by one of them a short time since of an African trader, with a cargo of slaves; they sent in and demanded a ransom of 27,000 dollars, and while the negotiation was going on, the captain of the captured vessel let the black-birds loose, and by this means destroyed a few of the gallant band, and so retook their vessel, and brought her in here.
Although who did what in this episode is not entirely clear, it seems highly likely that "the black-birds" refers to the enslaved "cargo" of the transport vessel.
Summary
The instances cited above yield the following chronology:
February 21, 1821: a letter seems to refer to enslaved Africans as "black-birds."
November 1836: An item from the [New York] Emancipator refers to "blackbirding" in the context of the slave trade without explicitly defining the term.
1840: A novel refers to slaving as involving laying traps "to catch blackbirds without wings."
November 4, 1852: A novel refers to working as a sailor aboard a slaver as "blackbird-catching."
April 25, 1857: A stevedore on a boat in New York City refers to a nearby slaving ship as a "blackbird."
April 17, 1861: A Philadelphia newspaper refers to a planned slaving run by a bark as "blackbirding."
If this chronology is accurate, it would appear that the term blackbirding evolved from an earlier notion of "catching [or getting] blackbirds"—a phrase in which blackbirds referred metaphorically to people of coastal Africa, presumably because their skin was dark.
