Broad Bodied Chaser (Libellula depressa)
If you see a dragonfly that is bright blue it is always worth a closer look just to make sure you do not jump to conclusions, the Broad-bodied Chaser is quite common and is one of the most likely blue dragons you will see.
Very different in colouring to the female which is olive green, the Broad-bodied Chaser is best identified through its ... broad body! The skimmers are much more pointed towards the tail and the Emperor is longer and even width along its long body.
This chap is fiercely territorial and it will often take over a small pond and defend it against all comes, apart from attractive olive green females of its own kind of course who are very welcome and greeted by being ceased and mated with in mid air!