We don't see much of the lovers' romance in "Song for a Dark Girl." We see no kind looks, no flowers, no chocolates, no kisses being exchanged. But the immense sadness the speaker feels upon the brutal death of her lover shows us more than any sentimental love scene could. She's more than sad—she's heartbroken.
There's no love in this poem whatsoever. The final metaphor says it all—love is a shadow. It doesn't exist for our speaker.
This poem shows that even something as universal as love can be destroyed by racism.