The Halfblack triplefin (Enneapterygius hemimelas), also known as the Half-black triplefin, Blackbelly triplefin, or the Green-tail threefin, is a species of triplefin blenny in the genus Enneapterygius. It was originally described by R. Kner and F. Steindachner in 1867. It is a non-migratory tropical blenny known from coral reefs in the western Pacific Ocean, and has been described from the Ryukyu Islands to eastern Australia. It has been recorded swimming at a depth range of 0-30 metres (0-98.4 feet).