Diction is the way an author or poet uses words.  In “The Weary Blues” hues uses the words to create the impression of blues music.

In the beginning of the poem, the rhythm seems to match the croon of the droning music.

Droning a drowsy syncopated tune, 
Rocking back and forth to a mellow croon, 
I heard a Negro play.  (lines 1-3)

Like the musician, the poem rocks back and forth in a simple melody.  Tune, croon, play, night, light, sway, sway.  The rhyme scheme is aabccdd.  By repeating the lines “he did a lazy sway” the music seems to sway.  The interjection of the exclamations, such as “O Blues!” and “Sweet Blues” is like the music coming to a crescendo of emotion.

The lines are short, and there are no stanzas at first, while he is playing.  This gives the poem a long, meandering quality just like the music.  When the second stanza starts, the song is wrapping up. 

The singer stopped playing and went to bed 
While the Weary Blues echoed through his head. 
He slept like a rock or a man that's dead. (lines 33-35)

At this point, the last line makes these lines prophetic.  Just as the song stays peacefully in his head, the poem will stay in ours long after we read it, because the words were carefully chosen to have a blues musical quality.