The Common Blue damselfly occurs throughout the UK and through most of Europe except the far north. The blue male, shown above in a heart with the pale olive green female, is bluer than the similar Azure Damselfly. On the second segment of the abdomen, he has a distinctive roundish black blob linked to a black line.
Common Blue male
There is a less common blue form of the female, shown coupled with the male above. She has less blue on her body than he does.
Some Common Blues (in several of the above photos) tend towards "whitelegged-ism" but the blue male White-legged Damselfly looks very different and has totally white legs.