No contrast is more natural than that between light and darkness; indeed, one dictionary definition of the very word "contrast" is the difference in brightness between light and dark areas of an image. As such, the light/darkness pairing has a rich history in poetry in literature. It's all over Shakespeare, for example; who could forget Macbeth's famous plea, "Stars, hide your fires; Let not light see my black and deep desires"?
Marley trods similar ground in "Concrete Jungle":
No sun will shine in my day today
The high yellow moon won't come out to play
Darkness has covered my light
And has changed my day into night
Where is the love to be found?
Sweet life must be somewhere to be found
Instead of Concrete Jungle
By adopting this familiar metaphor, Marley transforms what could have been a narrowly local story (about the particular miseries of life in the Jamaican housing project known to its residents as "Concrete Jungle") into something timeless and universal. The song can thus be just as meaningful to someone in England or America—someone who knows nothing whatsoever about the troubles afflicting the sufferers trapped in the middle of West Kingston's tribal wars—as it is to the sufferers themselves. And by equating light with both love and life (in the verse's closing lines), Marley invokes a Western ideological tradition as venerable as the Bible itself.