Pacific Rim Wiki wrote:"Kaiju Blue" is the blood of Kaiju. It is highly toxic to the Earth as well as humans. It instantly poisons the land it falls on, it also stains anything, from human skin, earth, and even keratin. If Kaiju Blue is inhaled, the victim will begin to cough up a blue vapor.
This is also why most weapons used by Jaegers are based on Bludgeoning or Cauterizing the Kaiju. Bunker busters would probably be okay as a last resort though.