I did not realize what these were until preparing to post them as "Argia species". I came across the illustrations in Lam (1), which match male and female quite closely. You can see on the male: eyes violet, upper part of thorax dark, lower part pale, abdomen black with blue rings, abdominal segment 10 black, segments 8 and 9 with blue chevron marks. On female, one can see the pale wedge splitting the black shoulder stripe and the striped abdominal segments 8-10. The last mark is quite distinctive, I think.