Here's another reference to dark skin color from Talley's 1922 collection:

STEALING A RIDE
Two liddle N----r boys black as tar
Tryin to go to Heaben on a railroad chyar.
Off fall N----r boys on a cross-tie!
Dey's gwineter git to Heaben shore bye-an'-bye.

-Talley,"Negro Folk Rhymes", page 188