Firstly it should be noted that I have only seen Southern Damselflies at Crockford Stream, and the presence of a bright blue damsel here is in itself almost diagnostic, as other similar species are rarely present.
Examination of the second abdominal segment of the male is required for a 100% positive identification, which looks like a small cartoon viking with a horned helmet (happy to accept better suggestions though!).
Males are a very similar blue colour to their Azure and Common Blue counterparts, yet have slightly more black colouration on the abdomen than both. The second abdominal segment is used for positive identification, showing a small black 'viking' figure. Females are almost entirely black, aside from green lateral lines dividing the abdominal segments and green eyes..