If you do not have it you should get Michael Moore’s Medicinal Plants of the Mountain West herb book (the orange one).
Here is an excerpt:
BLUE FLAG
Iris missouriensis
APPEARANCE: This is the predominant iris of the West and, in fact, the only species native to our area outside of central California northward. It is a typical iris, with long, smooth, lance-shaped leaves and a light lavender to bluish-purple flower slightly smaller than garden varieties. The plants form colonies and often cover whole meadows with their beautiful, early summer blossoms. The creeping root stalk is reddish brown and covered with the leaf scales of previous growth. The true Blue Flag of herbal usage and other medicinal practice is I. versicolor, the common blue iris of gardens, which is native to the eastern United States.