From the weakness Kairos experienced, he quickly realized that black blood was basically essential for survival, as insufficient amounts would basically cripple him. On top of that, apparently it was a source of food according to the others. Not to mention, black blood was used to evolve, and also the resource used to cast abilities provided by cores.
How it worked was that it would first prioritize being a source of food as well as health. From there, he would have the capacity to use the black blood for his abilities. If both of those bases were covered, giving him extra blood he couldn't carry, it would be used for evolving, which would also naturally increase the amount he could store.
In other words, black blood was what was needed for the equivalent of a health point bar, a hunger bar, a mana point bar, and even an experience point bar.
In a sense, it made things more simple, but also made them more convoluted.
