Some of the earliest blooming iris, in fact some of the earliest blooming flowers, are the iris commercially available as Rock Garden Iris or Dwarf Iris.
They often bloom concurrently with crocus here in the Catskills, and are comparable to crocus in size at flowering time, at 4 inches in height. Although listed in the catalogs as hardy to Zone 5, they are really hardier than that when grown in well-drained, gravelly soil and full sun, that is, rock garden conditions. Also, like crocus, they grow from bulbs, unlike other kinds of iris.
Almost all dwarf iris have been bred or selected from two species. The yellow varieties are based on Iris danfordiae, from Turkey.
Iris reticulata is the source of the majority of varieties in purple or blue. It is very widely distributed around the Black Sea (from Southern Russia through the Caucasus Mountains, then into Turkey) and there are many genetically diverse populations that vary greatly from each other.
Dwarf iris sourced from Iris reticulata no matter their base color, whether purple or blue, all have a conspicuous yellow stripe running down the center of the petal which acts as a billboard to pollinating bees: FOOD HERE! If you see a deep blue dwarf iris with no yellow stripe, it is probably a hybrid of Iris reticulata and Iris bakeri, a species that is otherwise not commercially available.