"Black reapers with the sound of steel on stones
Are sharpening scythes. I see them place the homes
In their hip-pockets as a thing that's done,
And start their silent swinging, one by one.
Black horses drive a mower through the weeds,
And there, a field rat, startled, squealing bleeds.
His belly close to ground. I see the blade,
Blood-stained, continue cutting weeds and shade."
This poem has the themes of sharpness/cutting/violence, blood/stains, darkness (black/shade), and hard labor. Throughout the poem, we see the blade as something that is very sharp and quite dangerous; we see violence (intentional or not). Next, we see two instances of blood (the RESULT of violence), one when the rat is bleeding, the other when Toomer describes the blade that has not stopped inflicting its violence, its cutting. Then, there is darkness: the word "black" comes up twice, and "shade" is the closing word. These words, while providing essential information such as the fact that the reaper and horse are black, create a running theme of darkness/somberness/melancholy in the poem. Finally, the theme of hard labour is evident throughout the poem, as it begins with work being done, and the work never stops. As we might notice, all these themes are representative of slavery in America.
While this may seem like a simple poem about slavery to us, since we learn about slavery in history class and can recognize its themes and conditions, there is more to be noticed in "Reapers." The title itself, and the word reaper (and concept of the reaper) has multiple meanings. Oxford defines "reaper" as a machine used in harvesting, a person who reaps (cuts), and, of course, the Grim Reaper (Death personified) himself/herself (OED). If we look up "reap," however, we notice that there are other definitions that we can apply: to receive as the consequence of one's own or others' actions, to take away by force, and to gain (OED). While the former set of definitions seems to apply to the slaves themselves, the latter set references/describes mostly slave-drivers, with the exception of the first definition. It is clear that all of these definitions can be used to describe both slavery itself and what is happening in the poem, which seems to be a short clip of slavery at work, or at least its institutions (hard/continuous work, blood spilling, etc.). Thus, which definition of "reapers" we use does not really matter that much; they all lead to the same place. The black workers are reapers in that they are machines, drones who continue to work no matter what; they are reapers in that they are people who do cutting; they probably reap the consequences of their actions (don't stop, or else...), though we don't really see this in the poem; and the final two definitions can even be referenced if we notice/remember that the slaves are working for slave-drivers who are trying to gain something, who will take away privileges if they don't get what they want.
Finally, I would like to point out the symbolism of the Grim Reaper definition, which I have saved for last. If we apply this definition—which, one will notice, by looking at OED, had been used by Longfellow well before Toomer started writing—the entire poem changes into a metaphor for the trials/issues that slavery presented, the darkness and violence. The Grim Reaper is the corporeal form of Death itself, and his/her appearance in a poem that mimics the institution of slavery is startling. It says that we can think of slavery itself as the reason for many deaths (of slaves), which is true. The idea of killing a mere rat, a tiny, insignificant thing, should remind us of the violence and ruthlessness with which slaves were treated in the time of slavery. Thus, with this definition, the slave-driver takes the place of the slave in the field; the slave-driver becomes the Grim Reaper, and then he/she reaps the rat, the slave. Now, we might notice a cycle: the slave reaps the weeds (re: the slave does the work), the master reaps the slaves (re: drives, punishes, and perhaps kills them), and finally the ACTUAL Grim Reaper is the one who takes all the souls away, white or black, to be judged or begin eternal life (an afterlife that will, one hopes, transcend slavery).
As we can see, the poem presents many parallels between death and slavery, and it also references some key issues that were, no doubt, close to Toomer's heart. Toomer seemed to be both speaking out against and referencing slavery (the importance of remembering it and making conditions better). That he was both "white" and "black" adds a deeper level of meaning to the poem, as he can be seen as being "on the fence" on the issues of slavery; he is on both sides, the white and the black (and North and South, if one wants to look at it that way). As a black man, he can identify with the plight of African-Americans, and as both a white and a black man, he has the authority and REASON to speak out against racism, to challenge it.