Although seventh chords generally contain a 1, 3, 5, and 7b notes (4-note chord), you can still get that bluesy sound with 3 of the 4 notes, as long as the 7b is one of those notes.
You can convert a major chord to a seventh in different ways. For example, you can take a 5-note and move it up 3 frets to get a 7b. Or you can move a 1-note down 2 frets to get the 7b. Either way, you get a 7th chord.
There is a file in the fretmentor blues materials that shows 7th chords, in different forms, that should be practiced. A good way to do this is to choose one chord per day and practice the different ways to make the 7ths of that chord.
