Frankly, from a survey of the issue, it doesn't seem to be a disguise so much as just a poetic way of talking about of what everyone knew was a brutal business. If anything, it played off of the 19th century hunger for ivory as well, making ebony and ivory prime exports from Africa in a euphemistic way that appeals to 19th century literary convention. It is a literary device, just another way of talking about the slave trade -- and because the particulars of the slave trade were well understood by the majority of literate people in slave-holding countries, it wasn't necessary to lay out the brutality because it was a given. If anything, it speaks to the remarkable level of acceptance white people had to the institution of slavery and why it took so very long for slavery to be abolished. It was easier to hear poetic allusions to the source of their wealth written by white contemporaries than to accept the truths of slavery and the Middle Passage written by formerly enslaved people and even by white eyewitnesses.

The term itself doesn't crop up often in scholarly literature, only showing up instead in memoirs, novels, and pamphlets of the 19th century. When it does arise, it's either as I said a poetic device or its used with a level of irony, comparing the human cargo to precious wood, which was shipped in similar conditions and fared much better. So I wouldn't call it a mask as much as just a familiar shorthand in the 19th century. While we like to imagine that people were very uncomfortable with slavery, by and large, that unfortunately was not the case and slavers did not feel the need to hide or mask what they were doing when they stole, bought, and sold human beings. They created an entire worldview and series of economic systems that justified what they did, wove it into the fabric of the settlement of North and South America, and it was taken for granted. Abolitionists had to speak code far more often than slavers did, especially in places where slavery was being contested.

I'd recommend checking out the work of historian Manisha Sinha, especially The Slave's Cause: A History of Abolition (New Haven: Yale, 2016) and The C**ounterrevolution of S**lavery: P**olitics and I**deology in A**ntebellum South Carolina (Chapel Hill: University of North Carolina, 2000) for discussions of writing about slavery and the slave trade, as well as charting attitudes towards the slave trade and abolition. French discussions of slavery also use the "ebony trade" a lot, so this is also worth checking out: Miller, Christopher L. The French Atlantic Triangle: Literature and Culture of the Slave Trade (Durham: Duke University, 2008).