Black mosquito larvae is the larvae of the native culex sp mosquitos ,these larvae are often seen wiggling around the surface of stagnant water where they feed on algae and bacteria, unlike blood worm they are air breathers and breath through a small tube on there abdomen, they make an excellent natural food source for fish and amphibians, especially newt larvae and aquatic insects,  providing a consistent food source ,the they can be stored for long periods if kept in a fridge or cold water.