- Family: Gossamer-winged butterflies – Lycaenidae
- Subfamily: Hairstreaks – Theclinae
- Wing span: Small, 27–35 mm (1.05–1.37 in.). Females larger than males.
- Wing upper side: Brown, edges reddish orange.
- Wing underside: Brown, white lateral stripes crossing forewing and hind wing. Hind wing with red strip along edge surrounded by black blotches. Two outermost margin blotches shiny blue.
- Habitat: Broad-leaved forests, and grazing land with trees and bushes.
- Flying time: Late June–mid-July.
- Overwintering form: Egg.
- Larval foodplant: Blackhorn (Prunus spinosa), bird cherry (P. padus) and raspberry (Rubus idaeus).
The black hairstreak is quite common in southern Finland, but it has disappeared from the west coast and the Åland Islands. Like the other hairstreaks it always keeps its wings pressed together when it is resting, making the upper side of the wings difficult to study. The species can be differentiated from the white-letter hairstreak, which it resembles, by the black blotches and broken white stripe inside the red strip that runs along the edge of the underside of the hind wings. Like all hairstreaks (except the green hairstreak), also the black hairstreak has a small spur on its hind wing.
Hairstreaks fly quickly and they meander. Males defend their territory in trees and bushes, waiting for females, who lay their eggs individually or in small bunches at the tips of the host plant’s branches.