There are numerous secular slave songs that refer not only to race but to skin complection.

Stephen Foster's "Old Black Joe" and James Brown's "Say it Loud I'm Black And I'm Proud" refer just to race.

But imo, the still widely known saying [in African American communities] "the blacker the berry the sweeter the juice" refers to skin color. This saying is excerpted from the following rhyme that is found in Thomas W. Talley's 1922 collection "Negro Folk Rhymes" [page numbers of all examples cited are from the Kennikat Edition, 1968]. Btw, Talley wrote that most of these rhymes were sung, and not recited as prose.

YOU LOVE YOUR GIRL
You loves'yo' gal?
Well, I loves mine.
Yo'gal hain't common?
Well,my gal's fine.

I loves my gal,
She hain't no goose-
The blacker 'an blackberries,
Sweeter 'an juice.
-page 95