- Latin synonym: Polyommatus semiargus, Plebejus semiargus
- Family: Gossamer-winged Butterflies – Lycaenidae
- Subfamily: Blues – Polyommatinae
- Wing span: Small, 24–34 mm (0.94–1.33). Females usually slightly bigger than males.
- Wing upper side: Females shiny brown. Males with shiny blue wings with thin black edges and dark wing veins.
- Wing underside: Light grey or light brown with white-edged black blotches. Scales in basal area turquoise.
- Habitat: Abundantly-flowered meadows, grazing land and roadsides.
- Flying time: Mid-June–mid-July. Second generation (mid-August–September) possible.
- Overwintering form: Caterpillar.
- Larval foodplant: Different Pea family plants (Fabaceae) e.g. clovers (Trifolium).
The mazarine blue is common in southern and central Finland. Males can be confused with the holly blue. On both butterflies the upper sides of wings are blue and the undersides have black blotches without any red patterning. The upper and lower sides of the mazarine blue’s wings are darker, however. Mazarine blue females look like the little blue, but are larger. Additionally, the line that is formed by black dots on the underside of the forewing is more curved than the little blue’s.
Males can wait for females in their hillside territories, but they also fly around. Females lay their eggs individually in the branching inflorescence of the host plant. The larvae have a gland that secretes a sweet, nourishing liquid that ants love to lick, so ants can often be found hanging around.