Part of Speech: adjective
1.Showing a casual and cheerful indifference considered to be callous or improper.
2. Happy or carefree.
Source: Oxford Dictionaries
Found in the line 2 of ” I Hear America Singing” by Walt Whitman
The word “blithe” have two meanings as an adjective based on Oxford Dictionaries. I believe it means “happy or carefree” on the poem, because when mechanics sing , they sing in a happy or carefree way. I do not see any improper or callous from the context.