- Latin synonym: Plebejus optilete
- Family: Gossamer-winged Butterflies – Lycaenidae
- Subfamily: Blues – Polyommatinae
- Wing span: Small, 23–29 mm (0.9–1.13 in.). Females usually larger than males.
- Wing upper side: Males shiny violet-blue, margin black. Females usually brown, violet-blue shade only at base of wings. In the north females can be almost as blue as males.
- Wing underside: Greyish white, with white-edged black blotches. Trailing edge of hind wing with two (or three) different-sized pairs of blotches, made up of red blotches (inside) and shiny blue ones (outside).
- Habitat: Especially bogs but also open moors.
- Flying time: Mid-June–mid-July.
- Overwintering form: Caterpillar.
- Larval foodplant: Caterpillars live different kinds of Heather Family plants (Ericaceae), in Finland especially bog bilberry (Vaccinium uliginosum). In other countries caterpillars also live on cowberry (V. vitis-idaea), bilberry (V. myrtillus) and bog rosemary (Andromeda polifolia).
The cranberry blue is common in all of Finland. It can be easily confused with the northern blue and the silver-studded blue, which also have shiny blue blotches on the underside of the hind wings. It is easy to tell apart the females of the species, however, as they have different patterns on the upper sides of their wings. The males can be differentiated from the other two species by the violet shading of the upper side of its wings. The orange-red blotches on the underside of both males and females cover a narrower area than that on the northern and silver-studded blue.
Males can wait for females defending their mating territory in bushes, but they are very mobile. Females lay their eggs individually on the leaves of the host plant.