Montresor does not consider himself an Italian. He is not participating in the "supreme madness" of the carnival. Everybody else on the streets is wearing a costume, including his proposed victim Fortunato. Montresor wants to remain as inconspicuous as possible because he wants to lure Fortunato off to his palazzo without being recognized by anyone. The black cloak and black mask will make him seem like a shadow--especially because Fortunato will be attracting all the attention with his gaudy jester's costume and the cap with the ringing bells. The biggest problem in this short story is getting Fortunato off a crowded street and down into the catacombs without being recognized as his companion when the missing man was last seen.
We gather that Montresor has lived in Venice for some time but still does not consider himself Italian from what he has to say about Italians in the following.
Few Italians have the true virtuoso spirit. For the most part their enthusiasm is adopted to suit the time and opportunity, to practise imposture upon the British and Austrian millionaires. In painting and gemmary, Fortunato, like his countrymen, was a quack, but in the matter of old wines he was sincere.
We can surmise that Montresor does not participate in the carnival because he does not consider himself Italian. His name is obviously French. Since he is not getting drunk and is not celebrating, it is plausible that he might hear about the bargains in Amontillado when other possible purchasers, such as Fortunato, were not attending to business.
The black cloak and black mask are also "in character" for a man like Montresor. He is a melancholy, solitary, brooding person, not unlike Shakespeare's Hamlet, who is dressed all in black when he first appears and who tells his mother:
'tis not alone my inky cloak, good mother,
Nor customary suits of solemn black,
Nor windy suspiration of forced breath,
No, nor the fruitful river in the eye,
Nor the dejected havior of the visage,
Together with all forms, modes, shapes of grief,
That can denote me truly. These indeed seem,
For they are actions that a man might play;
But I have that within which passeth show,
These but the trappings and the suits of woe.
I think it is appropriate for Montressor to wear black because it is a color that is often associated with death. People wear black to funerals. Women in mourning wear black to show that they are mourning the death of someone. The standard image of "Death" is a man with a big sickle and dressed in a black cape and hood. That's basically who Montressor is in the story. He is death. He lures Fortunado deep into the catacombs under the pretense that he is going to show his friend a valuable cask of amontillado. Montressor isn't angry about the task at hand or afraid of what he plans on doing. He shows no regret or sympathy either. He is simply the bringer of death.
The color black is also often associated with the "bad guy" in Hollywood. In my popular culture studies class, I teach my students to pay attention to the reoccurring film motif that evil is shown as "dark, foreign, and/or ugly" looking. Darth Vader wears all black. Darth Maul is foreign looking and wears black. The Volturi from Twilight wear all black. Scar's mane (Lion King) is the only mane that is black (instead of brown). In Captain America 2, the Winter Soldier wears all black. Ronan the Accuser (Guardians of the Galaxy) isn't human and is always shown wearing black. It's everywhere. Including Montressor from "The Cask of Amontillado."
Obviously Poe didn't mimic those films. But he did help establish the motif through characters like Montressor.