Here's my take on the verse:

One day the blackbird said to the crow
"What makes you love your farmer so?"

-snip-
The "blackbird" and "the crow" represent two Black people conversing. Using these characters, the Black composer/s of this song could reflect on the reality of conditions for Black people without getting in trouble. That is particular true since the second line "What makes you love your farmer so" is a later version of the line "What makes White folks hate us so."

That line was also given as "What makes White folks love us so". In that version - and in the version in which the referent "White folks" is replaced by "farmer" - "love" is an ironical, much safer, replacement word for "hate".

Here are some examples of those verses that refer to "white folks" rather that "farmer":

http://mudcat.org/thread.cfm?threadid=99864
Subject: RE: Lyr Add: Nashville Students Jubilee Songs
From:Q
Date: 13 Mar 07 - 01:38 PM

Lyr. Add: RAILROAD TRAIN
1.
Says dat blackbird to de crow,
Oh, my! hallelujah!
What makes dese white folks love us so?
Oh, my! hallelujah!
As I went in de valley to pray,
Oh, my! hallelujah!
I met old Satan on de way,
Oh, my! hallelujah!...
With music. Copyright 1884.
P. 13, J. J. Sawyer, arranger, 1885, "Jubilee Songs and Plantation Melodies, Words and Music." H. B. Thearle, Redpath Lyceum Bureau.

-snip-
Referent to the verse
"Oh said the black bird to the crow
makes the White folks hate us so?
Stealing corn has been our trade
Ever since the world was made.

[I quoted this verse as being from "Jim Crack Corn/The Blue Tail Fly" but Q corrected that by writing "From:Q
Date: 14 Mar 07 - 11:04 PM
The verse you quote from "Jim Crack Corn" (from Hippletoe) is from a song in Randolph called "The Crow Song," verses of it or similar songs appear in Odum and Johnson, Perrow, Talley, Cox, Harris and others, stories as well as songs... [in the thread" Lyr Add: Nashville Students Jubilee Songs", 14 Mar 07 - 11:04 PM

-snip-
http://www.gutenberg.org/files/27195/27195-h/27195-h.htm, Thomas W. Talley, Negro Folk Rhymes: Wise & Other Wise, 1922, p. 183


THE HATED BLACKBIRD AND CROW
Dat Blackbird say unto de Crow:
"Dat's why de white folks hate us so;
For ever since old Adam wus born,
It's been our rule to gedder green corn."

Dat Blackbird say unto de Crow:
"If you's not black, den I don't know.
White folks calls you black, but I say not;
Caze de kittle musn' talk about de pot."