One of the most common sounds in Blues is the Dominant 7th chord, (commonly called a 7th chord. Like the major chord, 7th chords can be derived from the major scale. A 7th chord is obtained by adding the flattened 7th degree (note) of the scale to a major chord.
Here is a common fingering for a C7 chord. Practice changing between C and C7 as shown in Example 12.
It is possible to build a 7th chord from any basic major chord. Here are some new ones to learn.