'Twas mercy brought me from my Pagan land,
Taught my benighted soul to understand
That there's a God, that there's a Saviour too:
Once I redemption neither sought nor knew.
Some view our sable race with scornful eye,
"Their colour is a diabolic die."
Remember, Christians, Negros, black as Cain,
May be refin'd, and join th' angelic train.
The word  sable  means dark, black. 
 It’s an interesting word choice since it’s more often used in reference to the animal or its expensive pelt. It brings into the poem connotations of market value and the inhumanity of chattel slavery.
On Being Brought from Africa to America