The blues is a foundational element of America’s vernacular and art music. It is commonly described as a combination of African rhythms and European harmonies. This characterization is inaccurate. Blues follows harmonic conventions that are quite different from those of European common-practice tonality. Blues does not fit into major or minor tonality, and it makes heavy use of harmonic intervals considered by tonal theory to be dissonant. But blues listeners do not experienced the music as dissonant. Instead, they hear an alternative system of consonance. In order to make sense of this system, we need to understand blues as belonging to its own tonality, distinct from major, minor and modal scales. The author argues that blues tonality should be taught as part of the basic music theory curriculum.