"Sky Blood" is a phrase War Boys use for their colorful signal flares, The term was first introduced in the 2024 prequel film, Furiosa: A Mad Max Saga.
History
Signal flares were used extensively by War Boys during the events of Mad Max: Fury Road. Flares are an ancient form of non-verbal communication.
During the events of Furiosa, a lost War Boy, dubbed "Lone War Boy," uses a red flare to signal Dementus' Biker Horde in passing. The War Boy refers to the flare as "sky blood." It is apparent that the War Boys are given limited education and knowledge, During the exchange, the War Boy has trouble understanding what the term"place of abundance" means and the History Man attempts to explain.
Dementus' Horde, suspicious of the flare gun, inspects it carefully before testing it by firing it into the ground, which causes a massive, red cloud to engulf the immediate area, turning everyone red. This is how Dementus came to be called "Red Dementus." Symbolically, this was used to relay that Dementus was entering into a phase of anger or rage as he began his conquest to seize the "Three Fortresses of the Wasteland."
In Fury Road
The flares that we saw in Fury Road were colored red and yellow. It is unclear what specific meaning each flare has. They were also used in combination. Presumably, the color combination code is worked-out by the War Boy before a run.
In Furiosa
At the start of the "Forty-Day Wasteland War," Immortan Joe and his "braintrust" have devised a plan to fool Dementus into thinking that they are going to Gastown. The Gigahorse, flying a red and black banner, leads a small contingent of vehicles. After Dementus takes the bait, the Citadel fires a pair of flares -- one red and one black -- to signal the Gigahorse.