Let's define two terms before we consider their use in the poem:
Diction: the choice and use of words and phrases in a text
Tone: the general character or attitude of a piece, carefully cultivated by the author
Now let's look at how Blake's diction conveys the speaker's tone:
"my wrath did grow" : Blake begins the transition from speaking literally, as an emotion can grow or wane, to figuratively, transforming the wrath into a plant. His metaphor develops from his choice of the word "grow".
"I waterd it in fears, / Night & morning with my tears": Maintaining the plant metaphor, Blake describes nourishing a plant with water while simultaneously insinuating that this water, or his tears, have a darker purpose and meaning: fear.
"And I sunned it with smiles, / And with soft deceitful wiles": Continuing with the plant metaphor, Blake introduces a common comparison between smiling and sunshine. However, in this case, Blake's sunshine, or his smiles, are false in nature, or as he says, deceitful.
"Till it bore an apple bright": The plant that Blake has been cultivating, his wrath, bears fruit, or is fulfilled (a common synonymous phrase would be 'comes to fruition'). The choice of an apple is particularly loaded with symbolic undertones given Blake's Christian background and the association of apples with sin in Christianity.
"And into my garden stole, / When the night had veild the pole": Here, Blake first puts the reader on his side by characterizing his foe as someone evil who steals at night. Metaphorically, the foe seeks contact with the speaker's fully realized wrath.
"In the morning glad I see; / My foe outstretched beneath the tree": The speaker's celebration is a bit disturbing given that he is glad to see his foe dead. The implications here are that the fruit of the wrath was poisonous, as negative emotions are wont to be, and that the speaker has triumphed over the foe.
Therefore: Blake's speaker in the poem "A Poison Tree" uses a tone that is dark but justified, a tone cultivated by the metaphor of the speaker's wrath as a poisoned apple, nurtured by the speaker's resentment and stolen by the speaker's foe.