Explain the meaning of "trodden black" in "The Road Not Taken".

The entirety of the line you are referring to in Robert
Frost's poem "The Road Not Taken is "In leaves no step had trodden
black." 

This line is an apt and pithy
description of the road the speaker is taking. The speaker of the poem has come across
two diverging paths in a forest area. He wonders which path he should take, and he is
sorry he cannot split himself in two so he can travel through both. Both have been
well-traveled, and he can tell from the evidence of the "trodden black." It refers to
the leaves on the ground, which have been stepped on by many people. The dirt on their
shoes and the slow decomposition of the leaves have turned them darker. However, the
speaker decides to choose the path that ends up being "less traveled," which means less
people have walked through it. He wants to come back and choose the other path the next
time, so he will have experienced both, but a part of him doubts that his future travels
will bring him back to this particular place. 

The
blackness of the leaves also contrasts with the initial "yellow" color that Frost
invokes in the beginning line. The wood he is traveling through is described as
"yellow." Because these are the only two colors mentioned directly in the poem, they
stand out as an interesting juxtaposition. Frost conjures yellow, a bright and sunny
color that seems to bring an image of a beautiful Autumn day. Then he conjures black,
dark and harsh against the yellowness of the wood's canopy. The intense variation lends
to the feeling of the poem: a bit lonely, bittersweet, and nostalgic. It seems to
capture the complex feelings of limited decision making, wanderlust, adventure, and the
metaphor of the traveler's dilemma.