The central way that this two stories can be compared thematically is through an examination of the way in which the narrator in "Sonny's Blues" and the grandmother in "A Good Man is Hard to Find" both are forced to confront and deal with their prejudice towards other characters in the story.
In "A Good Man is Hard to Find," we are presented with the character of the grandmother, who is shown to be an incredbly irritating, vain and somehwat exasperating character. She shamelessly manipulates her son and her grandchildren to get what she wants, and she likewise disobeys her son's instructions to leave her cat at home. She dresses for the journey thinking of what her body will look like if they have an accident, and she is desperate to keep up appearances. She seems blissfully unaware of her own failings, but it is only when, moments before her death, that she meets the Misfit that she seems to recognise how both he and her are similar in the way that the monstrous deeds that the Misfit commits are similar to her own attitudes and actions. This is symbolised when she protests that the Misfit is one of her "babies," just before he shoots her.
In "Sonny's Blues," the narrative strategy of having Sonny's brother tell the story shows us the way that his own attitude and understanding of his brother changes as the story progresses. The story begins with Sonny's brother's discovery that Sonny had been arrested for selling heroin, and his anger and shame at this is clearly displayed. However, as the story progresses, and when finally the narrator goes to hear his brother play his music in a club, he sees how his attitudes and feelings towards his brother had been prejudiced and wrong as shown by his own feelings when he listens to the music his brother plays:
Yet, there was no battle now, I heard what he had gone through, and would continue to go through until he came to rest in earth.
The brother, just like the grandmother, has a moment of epiphany when he realises just how prejudiced he has been and he moves towards an understanding of his brother and his suffering.