Forktail blenny - Meiacanthus atrodorsalis
More info for Nederlandstalig here :-)
https://www.aquainfo.nl/artikel/meiacanthus-atrodorsalis-vorkstaart-blennie/
To be honest I don't know why there is some color variation between some totally blue and some with the half yellow body. It may be related to sex or age.

"Meiacanthus atrodorsalis", the Forktail blenny, is a species of combtooth blenny found in coral reefs in the western Pacific ocean. This species grows to a length of 11 centimetres TL. This venomous species can also be found in the aquarium trade. It is also known as the Eyelash harptail-blenny, Poison-fang blenny or the Yellowtail poison-fang blenny.