Wheatley uses a simile in line seven when she refers to the Negros being black as Cain...Cain was marked by God which some believe was a mark of blackness...He was fleeing for his life after he killed Able; therefore, God marked him so no one would know who he was...
'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.
In line two, she uses a metaphor in that her soul is darkened or "benighted. She is expressing a comparison to he soul being black or unenlightened.
In line six, she is using hyperbole in that her race "is a diabolic die." Diabolic refers to Satan or the Devil...this exaggeration would be that her race has been colored by the Devil...
In line seven, she uses a metaphor in describing the train as angelic...this would be a reference to a heavenly train that takes a person to eternal bliss...