I'm not sure of the actual origins of the symbolism, but I do think that this article, "From the Self-Invention of the Whiteman in the Thirteenth Century to The Good, The Bad, and The Ugly" by medieval historian Madeline Caviness would be of interest to you. She at least shows that the signification of skin color with self-identity and, conversely, "otherness" in exists in the West even as far back as the High Middle Ages. And she does this by comparing medieval "media" -- stained glass, sculpture, manuscript illuminations with stills from the movie The Good, The Bad, and The Ugly with Clint Eastwood. Essentially, she's historicizing the two uses of race (or whiteness) within their unique contexts.

While I can't comment on 'the first' use of this symbolic dichotomy, I can say that it's certainly present in some of the earliest Christian writings. This contrast runs throughout the New Testament: witness John 8:12 (I am the light of the world. Whoever follows me will never walk in darkness, but will have the light of life.), 2 Corinthians 6:14 (For what do righteousness and wickedness have in common? Or what fellowship can light have with darkness?) and Thessalonians 5:5 (You are all children of the light and children of the day. We do not belong to the night or to the darkness).

This was a theme that Christian writers returned to repeatedly in the following centuries. According to Craig Koslofsky (Evening's Empire) it wasn't until the early modern period that more sophisticated interpretations of darkness, or at least night, emerged in European thought. Even then the dualism between light and darkness was very much found in Enlightenment thinking (the term itself is telling) and from there survived into modern discourse.