1. Life is in the blood, so we are prohibited to eat blood. That life is in the blood is a scientific fact that has not changed even today. So, the law applies.
2. God had given it to us upon the altar to make atonement for our souls, for it is blood that makes atonement for the soul, the Bible says. In the New Testament, God did just that, He offered His blood on the Altar. The blood belongs to the altar exclusively, and so we are called to abstain from it.