There are lots of metaphors in the poem. The eponymous mask is of course a metaphor for the facade that the poet feels he and other African Americans must adopt. The speaker also refers to the "bleeding hearts" that African Americans must hide. This is also, of course, a metaphor. Their hearts are not literally bleeding, but the metaphor helps to convey the pain that they feel.
Another literary device used in the poem is the repetition of the collective pronouns "We" and "our" to refer to African Americans. "We" is repeated seven times throughout the poem, and "our" is repeated five times. The repetition of these collective pronouns emphasizes the number of people who are persecuted and also implies a "them" in opposition to the collective "We."
There is also repetition in the poem of the declaration "We wear the mask." The final time this declaration is repeated, in the final line of the poem, it is also an exclamatory sentence. The repetition, culminating in the exclamatory sentence, implies the speaker's rising anger and frustration. The mask connotes the emotional oppression to which African Americans are subjected, and the speaker is of course angry that he and others should be subjected to such oppression.
The poet's use of rhyme also contributes to the impact of the declaration "We wear the mask" in the final lines of stanzas two and three. These two lines are the only lines in the poem which do not rhyme, and thus these lines—and the connotations associated with the metaphorical mask—are made to stand out.