Answer:
In Walt Whitman's poem "I Hear America Singing," the adjectives "blithe and strong" are used to enhance the effects of parallelism. The adjectives are used to describe the mechanics who are singing their carols as it should be.
The central theme of "I Hear America Singing" is the unique and positive contribution of every individual to the American identity. The line "Each singing what belongs to him or her and to none else" develops the overall theme that each song and task fully expresses an individual.
The repetition of the word "singing" emphasizes hope and a unified vision for America. The speaker reinforces in line 9 the metaphor of “singing” to mean individualism. The idea that each character is unique and has his or her own song, that each by virtue of his or her profession is essential to the whole of American society and culture, is expressly democratic in nature.