Blake's metaphor is definitely an important figure of speech in this poem. Additionally, Blake uses allusion and euphemism.
Blake alludes to the Bible's Garden of Eden when he describes the poem's tree. Just like Eve's tree in the Garden of Eden, the tree in the poem bears a fruit that is tempting and shiny, yet ultimately dangerous and the cause of the consumer's downfall. The apple in Blake's poem represents the speaker's unresolved anger, and the apple in the Garden of Eden represents knowledge and temptation. This allusion is particularly interesting because it brings up some interesting questions in the poem. Is Blake comparing Eve's innocence to the foe's innocence? Is Blake comparing unresolved anger to temptation and knowledge? Perhaps Blake is saying that giving in to someone's unresolved anger can be just as dangerous as giving in to temptation.
Blake also uses a kind of euphemism at the very end of the poem, saying that his foe is "outstretched beneath a tree." We can surmise that this is a polite way of saying that his foe is dead, because the speaker killed him.
There is more than one figure of speech, or type of figurative language, going on in William Blake's poem "A Poison Tree." Because the question states "the figure of speech," I'll try to focus on the biggest, most important bit of figurative language.
5 of the most common poetic figures of speech are metaphors, similes, personification, hyperbole, and symbolism.
"A Poison Tree" makes the most use of metaphor. The first stanza introduces the reader to a narrator who is angry. But he tells his friend about that anger, and the anger was let go (Frozen's Elsa would be so proud). The first stanza also mentions that the speaker is again angry, but this time he does not speak of it.
The next three stanzas begin the metaphor. His anger is compared to a plant or a garden. It gets watered at night and in the morning. He makes sure that it gets plenty of sun. The narrator explains that the plant (his anger) grew "both day and night" until it produced fruit. The narrator says that it is an apple, but it's simply a metaphor for a lure. The speaker uses that apple to lure his enemy into the garden and be poisoned. It's not clear if the apple was poisoned or the entire garden. Either way, the speaker's anger was nurtured to the point where he used it to kill someone. That's one messed up gardener.