What literary criticism theory can be used in critiquing Petals of Blood by Ngugi wa Thiongo?
I am not familiar with this work, but the beauty of the various literary theories is that many of them work for any work, you just have to ask yourself the right questions. For example, with feminist criticism, ask yourself, how are the women in this novel portrayed? What is the role of women in this society? Is there an intended and perhaps unintended message about the role of women in society as depicted by the characters in this novel? What would a staunch feminest say in reaction to this work?
With formalist criticism, ask yourself the literary-type questions. Who is the narrator? Why did the author choose that type of narration style? What is the structure? Why? What is the significance of the setting? Are there any symbols or motifs and what do they add to the meaning of the work.
The purpose of thinking about a variety of literary theories is that they become lenses through which you can look at a text in new ways and better understand what the author has accomplished.
In the references, I included a link to a web site that has brief overviews -- read through them and decided which are most applicable to this novel.
The information provided by the other editor is valuable! She is correct; you can apply any criticism to any piece of literature. What "type" is most applicable will depend on your end goal. That said, if I had to choose a type of criticism with which to analyze the novel, I'd focus on postcolonial literary theory. Postcolonial theory grew out of several other branches of criticism (Feminist, African American, Marxist, etc.). It focuses specifically on the unique issues faced by countries/individuals that have lived under colonial rule. As this novel is set in Kenya (just after the independence), postcolonial critical theory can help you understand some of the social, historical, political, and psychological issues presented therein.