In Marie Evans' poem "I am a black woman," the tone is one of perseverance and hope in the face of great pain and loss.
The speaker compares her life to a song, but one riddled with tears, written in a minor key, "minor" referring perhaps to "minority" and the struggle and denigration of that experience. However, although her song may not be heard in the daylight, it is humming in the night: this line is repeated again, inferring that it pulses on...a song that does not end—like the spirit of the black woman. The fact that a comparison is made to song indicates a sense of something positive.
There is nothing positive in the second stanza. The speaker talks of loss: her lover who leapt screaming into the ocean, gone forever (we assume); she gives birth ("my issue") while working in the cane fields; Nat (whoever he is to her) is lost, swinging—I would take this to mean he was lynched, while she cried countless tears at the pain of loss.
Next the speaker talks of hearing her son's scream (figuratively) for peace, all the way from Anzio, the site of fighting during World War II between the American and Allied Forces against Germany; she has learned about places like Da Nang (in South Vietnam, during the Vietnam War) and Pork Chop Hill (the scene of a raging two-day battle during the Korean War) "in anguish."
When the speaker mentions "the gas" and "trigger tired fingers," these also seem to speak of the trappings of war, that she may well have experienced vicariously through the descriptions and stories told to her by the men in her life.
In the last stanza, the tone changes again. The speaker identifies herself as she did at the beginning: "I am a black woman." However, now it would seem (with all the speaker has shared before) that the speaker is not one woman, but speaks for all black women, particularly those who have lost their men. "My mate leap screaming to the sea" may refer to the black captive whose man jumped into the sea rather than living enslaved amidst the horrors of a slaving ship. Giving birth working in the cane fields speaks to slavery. "Nat's swinging body" would refer historically to men lynched, especially in the South. "The gas" as well as the locations of men engaged in battle, speak of wars to which young black men traveled and lost their physical well-being or their lives.
All of these descriptions create a tone of suffering and loss. However, the theme of constancy that is introduced in the first stanza is continued again in the third stanza, along with hope and encouragement. The speaker notes that she is as tall and strong as a cypress tree, the wood of which is durable, tough. Her perseverance cannot be understood or measured: it defies comprehension, especially in the face of so much adversity. She will continue to thrive despite the "place and time and circumstance assailed." Her spirit is "impervious / indestructible." This spirit of woman encourages others who have also suffered to "Look on me and be renewed."
Evans' tone starts with the sense of steadfastness; she then presents the images of loss and death. However, she concludes with images of strength and hope. The speaker notes that she has survived: "she" being the black women of the past. She presents the essence of survival and imperviousness to the reader: look on what the past has done, and take heart; allow hope to fill your heart.