The boy/speaker in the poem is explaining his birth, his present state, and his eventual journey to heaven/God. He notes how he was born in the "southern wild" (presumably Africa). He then uses the symbolic associations of black and white (denoting darkness and light) in order to challenge the idea that these notions are connected to skin color. He claims that his soul is as white as the white child's soul. He adds that skin color (as compared to these symbolic notions, or not) is nothing more than a cloud. However, again playing with the comparisons, his black skin (cloud) will allow him to shade the English child from God's light until he (the white child) will be ready to embrace God's light.
The Little Black Boy begins by discussing the past: his birth and some lessons his mother taught him about God. During his life (the present), rather than black skin being associated with darkness, his dark tone is the result of bearing "the beams of love." Only after having learned this, will he be ready to enter the kingdom of heaven and bear God's light. He then foresees his death and afterlife, and promises to shade the white child until he can also bear God's light. At this point, they will be equals.
The Little Black Boy flips the traditional notions of light and dark as associated with good and evil. In this poem, his dark skin is not darkness or associated with evil; rather, it is the result of bearing light: love and God's light.