The males of the Common Blue Damselfly (Enallagma cyathigerum) are blue with black markings, while the females have a larger variation in their colouring.
Some females resemble the blue males, but they usually have more black patterning on their bodies. 
Other females are more of a brown or green brown color and do not resemble the males at all.

The Common Blue Damselfly can be easily mistaken for the Azure Damselfly (Coenagrion puella; in Dutch: Azuurwaterjuffer), but on the back and the thorax (the chest), the Common Blue Damselfly has more blue than black; for the Azure Damselfly it is the other way around.
Another important difference is that the Common Blue Damselfly has only one small black stripe on the side of the thorax, while all other blue damselflies have two.

Lac de Montriond, Haute-Savoie, France, 31 July 2016