The word "black" appears in the poem's third stanza in Frost's description of the two paths the narrator encounters:
And both that morning equally lay
In leaves no step had trodden black.
In terms of imagery, "black" suggests what the colorful leaves would have looked like had they been walked upon and ground into the probably moist soil. This reference provides a visual contrast with the scene the narrator does experience.
A thematic connotation also can be inferred from the passage. Both roads are covered in fallen leaves that are undisturbed by other travellers; regardless of which road the traveller takes, he will be the first to tread upon these particular leaves, creating a path of his own, perhaps for others to follow. Once he walks upon the leaves, he will leave behind evidence that he was here and made a journey of his own.