The Blues scale consists of 6 different notes. They are the 5 notes of the minor pentatonic scale, plus one additional note. The note added is the diminished 5th measured from the scale tonic. For example : adding to the C minor pent. scale : C - Eb - F - G - Bb - C the diminished 5th - Gb produces the C Blues scale : C - Eb - F - Gb - G - Bb - C In relation to the Major scale the notes of the Blues scale are : 1 - b3 - 4 - b5 - 5 - b7 - 1. The b3, b5 and b7 notes of the scale (for C Blues scale : Eb, Gb and Bb) are the so called blue notes of the scale. Here are the Blues scales in all 12 keys listed in Circle of Fifths order. C Blues scale C - Eb - F - Gb - G - Bb - C G Blues scale G - Bb - C - Db - D - F - G D Blues scale D - F - G - Ab - A - C - D A Blues scale A - C - D - Eb - E - G - A E Blues scale E - G - A - Bb - B - D - E B Blues scale B - D - E - F - F# - A - B F# and Gb Blues scales F# - A - B - C - C# - E - F# Gb - A - B - C - Db - E - Gb Db and C# Blues scales Db - E - Gb - G - Ab - B - Db C# - E - F# - G - G# - B - C# Ab Blues scale Ab - B - Db - D - Eb - Gb - Ab Eb Blues scale Eb - Gb - Ab - A - Bb - Db - Eb Bb Blues scale Bb - Db - Eb - E - F - Ab - Bb F Blues scale F - Ab - Bb - B - C - Eb - F

Wikipedia is (as usual) pretty helpful on this one: The Blues is a vocal and instrumental form of music based on the use of the blue notes. It emerged as an accessible form of self-expression in African-American communities of the United States from spirituals, work songs, field hollers, shouts and chants, and rhymed simple narrative ballads. The use of blue notes and the prominence of call-and-response patterns in the music and lyrics are indicative of African influence. During the first decades of the Twentieth Century, blues music was not clearly defined in terms of a chord progression. There were many blues in 8-bar form, such as "How Long Blues," "Trouble in Mind," and Big Bill Broonzy's "Key to the Highway." Idiosyncratic numbers of bars are also encountered occasionally, as with the 9-bar progression in Howlin' Wolf's "Sitting on Top of the World." The basic twelve-bar lyric framework of a blues composition is reflected by a standard harmonic progression of twelve bars in 4/4 or (rarely) 2/4 time. Slow blues are often played in 12/8 (4 beats per measure with 3 subdivisions per beat). By the 1930s, twelve-bar blues became the standard. There would also be 16 bar blues, as in Ray Charles's instrumental "Sweet 16 Bars" and in Herbie Hancock's "Watermelon Man." The blues chords associated to a twelve-bar blues are typically a set of three different chords played over a twelve-bar scheme: For example, if played in C, the chords would be as follows: C, C or F, C, C F, F, C, C G, F, C, C or G When the IV chord (F) is played in bar 2, the blues is called a "Quick-Change" blues. http://en.wikipedia.org/wiki/Blues