Common Blue damselflies start flying a few weeks after the Azure damselflies and I've noticed that the Azures fly close to the water, but the Common Blues fly along the grassy paths, and they are the ones you will see flying all around your feet as you walk along the riverside. The male is shown below, and it is all over blue. The Common Blue Damselfly can easily be confused with the Azure Damselfly which is also blue.

The main difference is the pattern on the thorax (shoulders). With the Common Blue this is an overall blue area, whereas the Azure has more black areas, and a large half length black line (the coenagrion spur). You can see this clearly in the image below. These patterns may just look random, but are surprisingly consistent on all damselflies.

Like many damselflies, the male and female are different colours. The green-brown female on the right in the photo below.