The related but uncommon Scarce Blue-tailed Damselfly superficially resembles this species but has a different position for the tail spot. At a glance could be confused with the two Red-eyed Damselfly species Erythromma najas and E. viridulum but are smaller, less robust looking insects and do not have red eyes.

Adult male Blue-tailed Damselflies have a head and thorax patterned with blue and black. They have a largely black abdomen with very narrow pale markings where each segment joins the next. Segment eight (S8), however, is entirely pale blue.

Female Blue-tailed Damselflies come in a variety of colour forms, with the colour darkening as the damselfly ages. The thoracic markings and the tail spot are violet in the immature form violacea, but salmon pink thorax and blue spot in the form "rufescens". When mature the female may be blue (like the male) in the form "typica", olive green thorax and brown spot in the form "infuscans" or pale brown thorax and brown spot in the form "infuscans-obseleta". Male and female both have bi-coloured pterostigma on the front wings. 

Larvaes are aquatic, and prey on small aquatic insects or other aquatic larvae. The adult damselflies prey on small flying insects, caught using their legs like a basket to scoop the prey up while flying, or insects taken from leaves.

At rest, the wings of most damselfly species are held back together, unlike dragonflies, which rest with their wings out flat.

Found in most types of water including brackish or polluted water where it may be the only species present. 
 

Larvaes occur amongst aquatic vegetation. Feed on micro-crustaceans and insect larvae. Most larvae emerge within one year, but small proportion wait a further year. Emergence on vertical stems of low growing plants close to water's edge. Often emerges in inverted position.

Flight period can be extended from March to October, depending on different locations.