Every moving thing that lives will be your food. I have given, even as the green herbs, I give all to you for food. However, you will not eat flesh with its soul, that is, its blood (Genesis 9:3–4).
After the flood that wiped out all the corruption in the world due to the perversion of the flesh of man and beasts, the environment has now changed. The earth is no longer covered by a protective shield of water, which made the environment ideal for vegetation, and this change, no doubt, caused many of the fruits and herbs that sustained human life to go extinct. In the garden, and all throughout the dispensation of conscience, man was given the green herbs and fruits of the fields to eat for his food (Genesis 1:29). Now the LORD is permitting the consumption of animals to sustain physical life.
The only restriction is that the life of the animal is not to be in it. This life, or literally soul, is in the blood. Therefore, humans are not permitted to eat an animal while it is alive, nor partake of its blood.