What does “But it's no more the negroes than I can fly” mean?

Solution 1:

The author is saying that it is ludicrous to blame the Negroes. "It's no more the Negroes than...." She compares believing "It's all on account of the Negroes" to believing that "I can fly." Then she hedges in a mocking tone by indicating that "I might fly out the window." And if she did fly out the window then maybe one could blame the Negroes.

Solution 2:

I will write an answer that is similar to that already contributed by @karyse, but slightly different.

I read the whole chapter your excerpt is taken from, and I also read a helpful review on Amazon.

The background for your excerpt is that the protagonist is feeling a bit hemmed in (claustrophobic) today, as her aunt is preparing for a women's get-together where the protagonist will be expected to engage in pleasant chit chat with people she never liked and hasn't seen in years, all while going through a major questioning of everything she thought she knew about the people who were close to her in her childhood. Simple version: She is disappointed to see how little real progress her Southern home town has made in overcoming the racist legacy left by slavery and Jim Crow. It is the 1950's, and whites in this little town are resisting the civil rights movement in ugly ways. Even her father has disappointed her, turning out not to be consistent with the anti-racism crusader image she had of him. She's depressed.

She sank into a deep armchair and considered how all occasions had made her poor indeed. My aunt is a hostile stranger, my Calpurnia won’t have anything to do with me, Hank is insane, and Atticus—something’s wrong with me, it’s something about me. It has to be because all these people cannot have changed.

Why doesn’t their flesh creep? How can they devoutly believe everything they hear in church and then say the things they do and listen to the things they hear without throwing up? … Everything I have ever taken for right and wrong these people have taught me—these same, these very people. So it’s me, it’s not them. Something has happened to me.

They are all trying to tell me in some weird, echoing way that it’s all on account of the Negroes… but it’s no more the Negroes than I can fly and God knows, I might fly out of the window any time, now.

As the previous answer, and the comments, have said, "it’s no more the Negroes than I can fly" means that the the idea that the African Americans in her town are the cause of her malaise, and her home town's malaise, is as absurd as the idea that she can fly.

The next part (which I realize you didn't ask about, but which the previous answer misinterpreted, in my opinion) is "and God knows, I might fly out of the window any time, now." This is basically free association. The protagonist (first person narrator) jumped from the phrase "I can fly" to imagining herself actually flying, and uses that image to express her feeling of claustrophobia.

Thus, the previous answer's interpretation of the second part of the sentence ("if she did fly out the window then maybe one could blame the Negroes") is incorrect, in my opinion.