Iris cristata

Wonderful plant. 

A member of the Crested Irises. It is a low growing, short stemmed (but longer than I. lacaustris), and vey floriferous plant.  Very pale iris blue. A native of North America where it grows in woods, along streams and rocky places. It is spectacular in flower in spring over quite a long timeframe. I have one in the full sun and one that is many years old in a dappled woodland setting in poor soil, beside a path, where is has formed an impressive spread and a mass of colour, where it is much happier than in the sun. Tidy looking leaves in the off season and they are nice and dense, which slows the weeds. Fully hardy and easy. 15cm high. 

Everyone loves it, but have you considered its close cousin Iris lacustris? If you want a tougher cookie in a rock garden.