Poe often uses visual imagery—a vivid description of things that we might then see in our mind's eye—to create scenes that we can visualize in our own heads. For example, at the beginning of "The Tell-Tale Heart," the narrator describes the old man's "vulture" eye: "a pale blue eye, with a film over it." The old man likely has cataracts, as this is common in the elderly and would create a kind of misty-looking veil over the pupil and iris, causing the eye to look pale. This eye seems almost otherworldly, unnatural, and the sight of it frightens the narrator very much.
Later, the narrator describes the old man's room as being "black as pitch with the thick darkness." Pitch is a viscous, very dark brown or black substance created from coal, wood, petroleum and other organic materials. To describe something as "dark as pitch" suggests how complete the darkness is—as though one is attempting to look through a swath of this opaque and black substance.