While Edgar Allen Poe's short story "The Black Cat" is long out of copyright, citation for it works the same as for most other written works. In this case, you should find the story reprinted somewhere, either online from a reputable website, or in an anthology. The citation would then follow the regular MLA format for citing a single work from a collection.
For example, if you were to cite "The Black Cat" from the website online-literature.com, it would look like this:
Poe, E. A.. "The Black Cat." The literature library. The
Literature Network, 2000. Web. 23 Apr 2012.
Since the website does not have a specific date when they published the story, you would use the earliest date from the site copyright; if you are unsure, you can use the date to/from, or 2000-2012 in this case. When citing sentences and paragraphs in the body of your work, you would simply use (Poe).
If you were citing from an anthology, for example the Complete Stories and Poems of Edgar Allen Poe, the format would be similar but with key differences:
Poe, Edgar Allan. Complete Stories And Poems Of Edgar
Allan Poe. Reissue Edition. New York: Doubleday
Books, 1984. Print.
You can see how the print edition is a little more concrete, since the book is not subject to sudden change -- however, the notation of "Reissue Edition" shows that this particular edition may have been revised. For in-text citations, use (Poe) but with the appropriate page number, e.g. (Poe 37).
The OWL at Purdue University has one of the most comprehensive MLA format guides available. Also see this Harvard website for information about legal citation issues.