Perhaps the most salient irony and black humor is in Montresor's mention of Fortunato's throwing up of the bottle "with a gesticulation" I did not understand--"He repeated the movement--a grotesque one." [but Montresor's actions are not?=irony] Then, when Fortunato asks if Montresor is not a mason, the narrator ironically replies, "Yes, yes." Of course, he is a brick mason this night that he buries Fortunato. And, herein lies the black humor.
After Montesor fetters Fortunato to the wall, he tells Fortunato to run his hand along the wall and feel the niter that he has ironically brought to Fortunato's attention earlier as though he were concerned about him. Now with black humor again, Montesor instructs Fortunato that he must notice it.
Once more let me implore you to return. No? Then I must positively leave you. But I must first render you all the little attentions in my powers.
This passage contains black humor, as well, since Montresor is merely toying with Fortunato, having no intention of letting the man return and since Montresor will, indeed, leave Fortunato--leave him to die and rot.
When Fortunato tries to tease Montresor out of his sinister intentions, there is much contrast between what is said and what is meant. Fortunato starts,
'But is it not getting late?....Let us be gone.' (Let us get out of this place.)
'Yes,' I said, 'let us be gone' (Fortunato will die/be gone, but Montresor will leave/be gone)
'For the love of God, Montresor.' (begging)
'Yes,' I said, 'for the love of God.' (making an oath: he will be revenged)