Blend curd, water, green chilli, ginger, salt and black salt on high speed in a blender until well blended and frothy. Add chopped coriander and mix well.
Refrigerate for 2 hours or until chilled.
Add oil to a tadka pan and allow to heat up on high flame. Once hot add mustard seeds and allow to splutter. Reduce the heat to low and add curry leaves and fry for 15 - 20 seconds or until crunchy. Turn off the heat. Mix well and add to the chilled buttermilk and serve.