The blues scale is made up of just six notes. It is built upon the minor pentatonic scale (which has 5 notes with the 6th as the octave) and contains an extra diminished fifth note, known as the famous "blue note".
To understand how this scale works, firstly you must understand the pattern of a minor PENTATONIC SCALE which looks like this in first position:
Here are the fret positions of the pentatonic scale in open position on tab:
These 5 notes are used to create melodies, riffs and solos for songs in the key of C Major or A Minor. Obviously, these 5 notes repeat all over the fretboard and can be played in various positions. This fretboard illustration shows all of the 5 notes right across the fretboard:
ADDING THE BLUES NOTE
The blues scale contains the same 5 notes as the minor pentatonic scale above but with an additional diminished 5th note which is a semitone lower than the 5th note. It can sound confusing but try and get used to the scale pattern for now. The theory behind it will make sense later.
In this case, the diminished 5th note appears in the form of an Eb note (which is a diatonic note and can also be read as D# but let's not confuse matters any further) This is what the blues scale looks like with that diminished 5th note:
As illustrated, there is now an Eb at position 4 and the E note has now moved to the 5th position with the G in 6th. Check out the tab below to see how the notes are played across the strings in first position.
Just like the pentatonic scale, this blues scale can be played all over the fretboard so use this illustration to play around with some ideas for creating your own solos: