The poem begins with a quiet, ordinary scene: branches hanging over some stairs, a grassy spot, a tree stump lit by the sun. Nothing seems wrong. Then the view shifts. From a distance, something in flight turns into a woman dressed in black, bigger than the maples, standing guard over the place like a ghost protecting buried treasure. The speaker never says what the "flight" really was, and the poem suggests that this blur of shape and distance is exactly how fear takes hold.
The speaker and the others with them feel real terror and choose to walk the long way around. The last line is the sting: years later, they no longer remember why they avoid the stairs. The habit has outlived its cause. The mood moves from calm to dread to a quieter, uneasy sadness, since a pleasant, sunny spot has been lost to an old fright no one can explain. The title, "Monochrome," fits the black figure and the way a single fear can flatten a whole place into one color.