Well since all three biographers segue to Run, Nigger, Run we don't have to guess or interpret. They can speak for themselves:

“Apparently, the “Santa Monica Examine 'Er” wasn't an isolated incident, either. On October 20, 1937, Woody received a letter from a listener that read in part: “You were getting along quite well in your program this evening until you announced your 'Nigger Blues.' I am a Negro, a young Negro in college, and I certainly resented your remark. No person… of any intelligence uses that word over the radio today.” Woody was mortified. It was a word he'd used casually all his life. It was a word he'd used lightly, jokingly, without ever quite realizing its full implications. He took to the air immediately with an apology. He read the letter aloud, promised not to use the word again, and ripped all the “nigger” songs out of his book.

Woody's political naïveté did, at times, seem boundless…”
[Klein, p.95]

Note: Student Howell Terence's letter is in the archives. Klein doesn't give his source for Guthrie's response.