Watch the Addressing The 3rd online guitar lesson by Jeff McErlain from Blues Survival Guide: Rhythm Edition

ATTENTION: This may be the single most important section of this course! The key note to the blues is the 3rd, and this is where it gets tricky and super cool. Let's talk scales for a second, the first scale we guitar players grab on a blues is the minor pentatonic scale. But have you stopped to consider that the I chord in a blues is major? So we are playing a minor scale over a major chord, in any other situation this would be a serious no no. But in the blues it sounds great, if you don't sit on that note that is. So we tweak it a bit to allude to that major third that is in the chord. We want to bend it a 1/4 step and usually resolve it. Generally we don't want to spend too much time on that note. This bend gives us that characteristics flavor we associate with the blues. We can also fret that note and sound the major third. This sound is bluesy as well but much much brighter. Not bad, but certainly a choice that has it's place.