The first line states an idea. Blues is poetry. There is a distinct rhyme scheme and rhythmic scansion to the three-line form of text. Usually somewhere around 12-14 syllables in each line, but this can vary substantially. The rhyme scheme features an end rhyme between the first and third line of text.
Blues is a musical form.
Three chords are used, with a different chord area emphasized in each line of text. Often, each section is divided into two measures of sung text and two measures of instrumental fill like a comment on the words. Arising out of the melding of European and African concepts of pitch, they create a tension of being in between pitches of traditional European scales. The blues scale includes a flatted third, fifth, and seventh degree. Often there will be sliding to and through these notes.