It seems pertinent to give Colcord's lines and words about slaving, and about "Blow, Boys, Blow" since they are discussed somewhat obliquely here.
"A group of famous old shanties had their origin in the packet-trade with Liverpool, which developed soon after the close of the war of 1812."
Her primary version, following that quote, begins "A Yankee ship came down the river" (name of river not specified). The captain was "Bully Hayes." Following the chantey, she writes of the chantey being taken over by "the 'packet-rats' of the Western Ocean, and celebrated the brutalities aboard the Atlantic liners" and "Doubtless all of the well-known masters and mates .... have heard themselves picturesquely described in this shanty; but it is Captain Hayes, who was lost in the Rainbow in 1848, whose name seems to have survived."

Following the chantey and musical score, she says, ""Blow, Boys, Blow" started life as a slaving song, the opening couplet being "A Yankee ship on the Congo River, ...."
She gives another couplet, "dating evidently from the Civil War ....
What do you think she's got for cargo? Old shot and shell, she breaks the embargo."

Obviously Colcord regarded the chanty as belonging to the period after the War of 1812, and up to and including the Civil War, and that it included versions concerning the continued trade in slaves despite US law.

Yankee slavers (mostly sailing from the southern ports but also from the northern ports but with other destinations stated rather than the African slave ports) continued to operate during the entire period because of the enormous profit in the slave trade, undeterred by the US law. Some were stopped by the US Navy (see previous post) but that didn't stop the trade.