This poem compares two very similar dreams. The dreams exist on a literal and figurative level. The key to understanding these similar lines is the lines they correspond to. The first stanza is less sure, and more childlike.
To whirl and to dance
Till the white day is done.
The second stanza is stronger, fiercer, more enthusiastic and more desperate, but also more self-assured.
Dance! Whirl! Whirl!
Till the quick day is done.
In the first stanza, the day is "white" and in the second one it is "quick" instead. These two lines lead to the juxtaposition of the last lines of each stanza. He used "dark" in the first stanza, and "black" in the second. He is older, stronger, and fully accepting and proud of his dream- and his race.