The poem begins with the speaker talking about his upbringing and his relationship with his mother. He explains that he was born in "the southern wild," an allusion to Africa (which lies far to the south of England). Consonance and assonance in "mother" and "southern" link these words together, lending the poem a kind of sing-song, bouncy rhythm from the start.
Line 2 then begins to develop its antithesis between blackness and whiteness. The boy is aware of his skin color, but professes that his "soul"—that is, who he is on the inside—is "white." The poem is trying to relate the well-established religious symbolism of light and dark (good and evil) with the skin color. By stating that his "soul is white" the speaker argues that his soul is faithful and pure, regardless of what he looks like. The exclamation mark after "O!" creates a pause, or caesura, in the line that marks the strength of the speaker's feeling.
In lines 3 and 4, the boy compares himself with an "English child." The other boy is "white as an angel," making it seem—given the way that Christianity links whiteness/light to goodness—that he is somehow more Godly than the speaker himself. The white boy looks angelic, because angels are so frequently depicted as being white and shrouded in a bright aura of shining light. The common rhyme of "white" with "light" in this stanza reinforces this connection.
If skin color really is an indicator of godliness, worries the speaker, then his own dark skin makes him look as if he is "bereav'd of light" (lacking in the light of God). Line 4 uses heavy /b/ alliteration through "but," "black," and "bereav'd," defining the line with one particular attribute in a way that echoes how the boy is defined by his skin. "Bereav'd" is an important word choice because it suggests a sense of loss and sorrow. Though the reader learns little of the speaker's circumstances, his perception of inferiority—or of the potential to be judged as inferior—could be based on his experiences of the world thus far (the Atlantic slave trade, for example, was in full swing).
The form established in this stanza will continue throughout the poem. Each stanza is a quatrain, meaning it has four lines, that fall in an ABAB rhyme scheme. The meter is iambic pentameter, meaning each line has five poetic feet in a da-DUM rhythm. All of these attributes make the poem seem rather simple and familiar, which is fitting given that its speaker is a child.
Ironically, and likely unintentionally, even in the first stanza the poem starts to show some of the racist attitudes that it attempts to argue against. For example, Blake associates Africa with uncivilized wildness and a lack of moral and social refinement. These types of arguments were often used to justify the conquering of nonwhite peoples and the enslavement of Black men, women, and children. It's important to keep such context in mind while reading the poem, which reflects many of the racist ideas of its time even as it pushes for equality.