His blood boiling was only visible in two situations: awakening and gambling with his life.
There was a saying in the demon descendants' community: "When someone shows his red eyes towards you, don't hesitate to kill him."
Blood boil was accompanied by scarlet eyes, which meant that the other side was going to stake their life. When one met a red-eyed opponent, one must not have any pity.
Another situation was when someone experienced an awakening. The blood boiling was because the ancient demon genes hidden in the body had been activated.
There were obvious differences between the two situations, such as the red skin color of Li Xianyu at this time. This was a phenomenon only accompanied by awakening. During the first awakening, the ancient demon genes would be activated, and these aggressive genes would modify the human gene sequence. One would feel like one had been reborn.