Male and female Common Blues have different coloration patterns. The upper sides of the wings of the male are blue with a touch of mauve, which deepens and softens the colour, with a narrow edging of black around the outer margins. The vein markings are a light blue but become dark towards the outer margins, merging with the narrow black edging. The female is normally brown on the upper sides of the wings with the blue scales present only in the area next to the body. She has a series of orange crescents with black spots on the outer margins - more pronounced on the hindwings. The patterning on the undersides of both male and female is the same, but there is a slight difference of basic colour: the male being pale brown-buff diffused with bluish grey, while the female is more definitely buff-coloured. The outer edges of the wings are marked with orange crescents with black dots and outlined in black, the orange being a little more pronounced on the female than the male. Immediately behind this are a curved row of black dots surrounded by white, (the arrangement of which can be used to distinguish female Common Blues from the Brown Argus) and a further row following the body line; the hindwing has a white saddle shaped mark and a white flash leading down to the fifth orange segment.
The adult insect from the first brood (i.e. the second generation insect) emerges at the end of July and is on the wing well into September - if weather is good, possibly to the end of the month. Eggs laid from this second generation hatch into caterpillars which go into hibernation after the second moult (third instar) and remain dormant until the spring when the cycle is completed. The first generation imagos are on the wing in May and can be seen until mid-June. If the year is exceptionally good and there are three generations, then the Common Blue could be seen more or less continuously from early May to late September.
One of the favourite nectaring plants is Marjoram and also Scabious.