This species apparently has no accepted English name, so I made up this one that translates its Scientific name. Four subspecies are recognized with the population on Amami assigned to O. m. ryukyuense. Taxonomy is confusing but my understanding is that this is a split from the Blue-tailed Forest Hawk (Orthetrum triangulare), sometimes called "Ohoshiokaratonbo" in Japanese. The dark bases to the wings start out black, but turn blue with age as seen here. This is a male. Females look quite different. Canon PowerShot SX60 HS.
Reference:
Yong, H. S., Lim, P. E., Tan, J., Ng, Y. F., Eamsobhana, P., & Suana, I. W. (2014). Molecular phylogeny of Orthetrum dragonflies reveals cryptic species of Orthetrum pruinosum. Scientific reports, 4, 5553. doi:10.1038/srep05553