No doubt about it, there's a little bit of suffering in "Yet Do I Marvel." We've got blindness, death, starvation, and endless physical labor. Okay, so a big bit of suffering. And all of this from a benevolent God, no less. Talk about sour grapes! Suffering is both a contradiction of God's goodness and a source of inspiration for the speaker to express his wonder about being a black poet in America.
The speaker uses examples of suffering in the world to work out his ideas about why God, at times, seems like a physical trainer from hell, putting people through death, hunger and meaningless labor. C'mon, feel the burn!
Although God is good, He has a funny way of showing it; but not every source of suffering is necessarily a bad thing, and the speaker turns suffering into a source of wonderment about his identity as a black poet in America.