The common blue damselfly emerges slightly later than the two most similar species in Norfolk – the azure and variable damselflies – but may be seen with them over a period of several weeks. It tends to be seen over open water, whereas the two other blue species are more often seen in waterside vegetation. At a distance the male common blue appears the most intensely blue of the three species, looks solidly built and has an obvious blue blob towards the end of his tail. Seen perched, it is possible to see that on S2 (the second segment of the abdomen) the male common blue has a distinctive round black blob with a slim stem joining it to the hind edge of the segment. S8 and S9 are completely blue.
Female damselflies are even harder to identify than males. However, the female common blue, who is usually a dull fawn in colour, has black rocket-shaped marks on the segments of her abdomen.