The Blood King's possession of the Ancestor God's blood was his secret to begin with.
Yi Yun came from the Tian Yuan world and having heard Jiang Xiaorou's description of the Blood King's growth, he surmised that the Blood King might possess the Ancestor God's blood.
It was unlikely the Ruins Emperor knew of the secret.
However, when Yi Yun's pills were formed, the phenomena which resulted included a scene of Human, Fey, and Celestial fusion despite it lasting only for a few seconds.
However, it was likely that the Ruins Emperor had noticed something from the auras of the phenomena. He had likely seen the Ancestor God's blood before and because of that, he could sense the Ancestor God's aura from the phenomena!
If the Ruins Emperor could tell, it was likely the other two patriarchs did as well!
