- "...The acid factor of the Kaiju blood creates a toxic phenomenon known as Kaiju Blue."
- —TV Reporter
Kaiju blood is phosphorescent in nature and highly toxic to the Earth as well as humans. It contaminates the environment where a Kaiju is either injured or killed. When the body of a Kaiju begins to decompose, its blood becomes a vapor (referred to as "bloodmist" by the K-Science division) that allows it to spread throughout the city.
The nature of the blood allows it to stain anything, particularly human skin. Though humans use breathing masks as a way of protecting themselves, they can still be infected by the bloodmist.[note 1]
In the early stages of the war, HAZMAT teams were dispatched immediately after the Kaiju threat was eliminated to clean up the area and remove the toxic remains. But, as the war progressed, and more Kaiju began to attack, the Kaiju's bodies began to be left where they fell, and the "Kaiju Blue" was left to run.
However in many cases black market dealers in Kaiju parts, such as Hannibal Chau, swooped in after a Kaiju had fallen and neutralized the "Kaiju Blue" to allow them to harvest various body parts. One such case is the body of the Kaiju Reckoner. After it was defeated at Hong Kong in 2016, its carcass was left to rot prior to its conversion into a temple for a Kaiju Cult in the Bone Slums.
In a effort to reduce blood and gore to appeal to a wide audience, Guillermo del Toro and the production designers for Pacific Rim decided that the blood of the Kaiju would be blue, particularly to create a color contrast in the Baby Kaiju set in Hong Kong and how it would spread in an environment "like an evil mold".
Costume and Production designer, Kate Hawley and Andrew Neskormny referenced photography of an Asian fabric factory stained with indigo dye as means to explain how people were affected by breathing in Kaiju Blood in its vapor form.
- ↑ During pre-production Costume Designer Kate Hawley sent Production Deisnger Andrew Neskormny a photo of an Asian factory where indigo fabric was being dyed. "Everything gets stained," recalls Neskormny. "People's clothes, their fingers, their nails, just everything. I said 'Oh, yeah, that looks geat. We've got to do a bit of that.'" Hawley took the idea even further. "The idea is that that the blue blood had got into people's bodies," she says, "and you see bits of blue coming through their breathing masks, like a subtle little poison."
- ↑ "Kaiju Blue was bad news, a kind of shock reaction the human body suffered when recently dead kaiju started to off-gas toxins in the hours after they died. It killed a lot of people when they went down in populated areas. Today it would only kill fish, and who knew if they got Kaiju Blue, anyway?"