“And both that morning equally lay/In leaves no step had trodden black” suggests nobody had walked on the leaves yet or left tracks.  However, the speaker’s choice of path or life journey will inevitably take him to darker regions as well as to those that are lighter.  It may also connote some of the more somber moments he may experience when he wonders how his journey might have been if he had taken the other path.

This latter may also cast light on his “sigh(ing)” in the fourth stanza, when he will look back on his life from a distance of years.   A sigh may have multiple meanings, but one that is invariably present when we reflect on time past is our acceptance or lack of acceptance of the inevitable that has ensued as a result of our decisions.  The significance of the sigh is what will make the “difference” of his having “black(ened)” one path rather than another.  However, as yet, the entire story of his journey is still incomplete which lends universality to the question posed by the poem.

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.