[7] The dorsal side of the wings is an iridescent lilac blue, bright violet-blue, or almost hyacinth-blue with a thin black border.

[4] Both sexes have a row of red or orange spots along the edge of the hindwing and extending onto the forewing, though they are generally fainter there, particularly in the males, where they are sometimes missing altogether.

[4] Other similar species are:- The caterpillar is small, pale green with yellow stripes and, as usual with Lycaenid larvae, rather slug-like.

[8] The common blue butterfly is found in Europe, North Africa, the Canary Islands, and east across the Palearctic to Northern China.

[10][11] It was discovered in Mirabel, Quebec, Canada, by Ara Sarafian, an amateur entomologist who observed the butterfly from 2005 to 2008.

[12][13] A study from Montreal, Quebec, Canada showed that the common blue is most abundant in areas with greater urban land cover and where their preferred larval host plant, bird's foot trefoil (Lotus corniculatus), can be found.

[12] These butterflies inhabit flowery or grassy places, warm and cool, open or wooded areas and at all altitudes up to high alpine meadows at an elevation of 0–2,700 m (0–9,000 ft) above sea level.

[7][4] It mostly resides on chalk or limestone grassland, but also in smaller numbers in woodland clearings, meadows, heathlands, sand dunes, along railway embankments, and under cliffs.

[9] Previously, P. icarus was a very common species that occupied Europe and Asia, and was one of the most widely distributed butterflies in Britain.

This could be because 46% of the total land area covered by the butterfly's preferred host plant, Lotus corniculatus, has also been lost since 1901.

Recorded food plants are Lathyrus species, Vicia species, Vicia cracca, Oxytropis campestris, bird's foot trefoil (Lotus corniculatus), Oxytropis pyrenaica, Astragalus aristatus, Astragalus onobrychis, Astragalus pinetorum, black medick (Medicago lupulina), Medicago romanica, Medicago falcata, common restharrow (Ononis repens), wild thyme Thymus serpyllum, lesser trefoil (Trifolium dubium), Trifolium pratense and white clover (Trifolium repens).

[6][15] Common blues sequester flavonoids from their host plants and allocate these pigments that are UV-absorbing into their wings.

Flavonoid sequestration is much more effective when coming from natural host plants than from experimentally offered diets.

This richness in females may increase visibility, but could also confer information about feeding history, and consequentially the quality of potential mate.

[17] Flavonoid sequestration is an important component of intraspecific visual communication and sexual signaling in Polyommatus butterflies.

The chrysalis is olive green/brown and formed on the ground, where it is attended by ants of genera Myrmica, Lasius, Formica, Plagiolepiss, [15] which will often take it into their nests.

The larvae of P. icarus are oligophagous, meaning they utilize a range of host plants in the family fabaceae, as well as have a mutualistic relationship with ants.

Visual systems in butterflies are highly diverse and their color vision abilities have only begun to be explored.

This enables the common blue to see color in the green part of the light spectrum extending up to 560 nm (2.2×10−5 in).

In Greek mythology, Icarus was the son of the master craftsman Daedalus, the architect of the labyrinth of Crete.