Cmaj7 - Fmaj7 - Gmaj7 chord inversions - the diagrams of these chord inversions could be used as "substitute chords" to play a jazzier style of Blues in Am. For Am7 use Cmaj7 - this gives you an Am9 chord with no root. For Dm7 use Fmaj7 - this gives you a Dm9 chord with no root. For Em7 use Gmaj7 - this gives you an Em9 chord with no root.