Normally in pop and rock songs, eighth notes divide each beat into two equal pieces. The eighth notes create the familiar “one & two & three & four &” feel that were used to in rock songs. On the other hand, blues guitar uses a swing feel, where each beat is divided into three pieces. Instead of “one & two &,” we get “one & a two & a three & a four & a.” Breaking the beats into three pieces creates what are called eighth note triplets. Since there are usually four beats per measure in the blues, you are usually playing four groups of three.
When you are learning how to play blues guitar, you want to practice strumming a chord like an E7, which is a shorthand way to write E dominant 7, with a swing feel. You should practice strumming down on the strong beats, those that fall on the one, two, three, or four, skip the