What theme does the poem create about the Modernist notions of truth?
The original question had to be edited. I would suggest that the poem's echo of the Modernist construction of truth lies in truth being seen as far from absolute. Truth, as the Modernist would define it, is a fragmented quality. This is reflected in the fundamental "shift" intrinsic to Modernism. This "shift" is one in which individuals no longer find the same sense of absolutism and totality that they used to find in the world:
On or about December 1910 human nature changed. All human relations shifted, and when human relations change there is at the same time a change in religion, conduct, politics, and literature.
The shift in human relations is where the understanding of truth also was altered. This development led to a Modernist vision of truth that is fragmented and not whole.
Frost's poem echoes this. When the speaker articulates that there was "something" in the way of truth found, it is undercut and denied. When the speaker describes this reality as "Blurred it out, blotted it out," it is clear that the vision of truth articulated is not an absolutist notion of understanding. The "shift" that is such a part of Modernist thought is seen in how the poem understands truth. The mere title of "For Once, Then, Something," alludes to a vision of being that is fragmented, not totalizing. This lack of transcendence is a part of Modernist notions of the truth.