You may want to look at the conventional rhyme scheme of both of these poems. The Countee Cullen poem has an abcb rhyme scheme in which the second and fourth lines of each stanza rhyme, and the Edward Arlington Robinson poem has an abab rhyme scheme. Both poems also have four-line stanzas. The structures of both poems are traditional.
In both poems, the irony comes from the endings of the poems, in which what the reader expects is upended. In the Countee Cullen poem, the narrator's visit to Baltimore, which starts with the narrator feeling so happy, ends up with his receiving a racist insult that literally colors the rest of his experience in that city. In the Edward Arlington Robinson poem, the person whom everyone admires kills himself in the end. Both poems end in a very different way than what the reader first thinks, and this creates a sense of situational irony.
How do you think the poems' conventional structures enhance this sense of irony? It is perhaps because the reader expects something straightforward but winds up with a poem that, while it has a traditional structure, is truly unsettling in its content.