They could have learned it as easily as during a winter famine. The smallest and least helpful of the group was eaten so the rest could survive. Think dinner party. The child freaked out, was dragged from their parents arms and slaughtered painfully, they were starving. They are the child and drank the blood due to lack of drinkable water. Then they learned what the fearful blood could do. It became a taboo. this is just speculation but the practice could stem from before judean history. From the men with out god. Pre-adam.
Actually there is a natural curb on the practice, those who consume their own species develop crusfelt jakobs disease. It the human version of mad cow. It is sped up when you eat those closely related to yourself.