Despite possibly already knowing (through their patriarchal ancestors: Abraham, Isaac, and Israel [Jacob]) God tells them as a part of the new covenant he was making with Israel to never consume any blood:
Leviticus 3:17
“This is a lasting ordinance for the generations to come, wherever you live: You must not eat…any blood.”
God also tells what he probably also instructed Noah to do (even though not directly recorded): he tells Israel when they kill an (clean) animal to eat the meat for food, to first try to drain almost all the blood out of the body (most likely by slitting the throat or cutting off the head entirely) to pour out all the blood onto the ground them cover it up with dirt so the meat they eat will be (pretty much) blood free:
Leviticus 17:13
“‘Any Israelite or any foreigner residing among you who hunts any animal or bird that may be eaten must drain out the blood and cover it with earth,…”
God directly gives at least some of the reasons for why he forbids Israelites to eat blood:
God reveals on major reason to not eat the blood is because that blood is the core of why a living soul (that is a living animal/human) is alive! As God says:
Leviticus 17:11-12
“...the life of a creature is in the blood,…Therefore [this is why] I say to the Israelites, ‘None of you may eat blood, nor may any foreigner residing among you eat blood.’ ”
The life/lifeforce of an animal (and human) is the blood and in the blood!
For this reason, blood was considered sacred or holy in a sense: so much so that blood from a sacrificed animal was the only way to properly atone (take away or make right) for human sins since the pouring of the animal’s blood on the alter symbolized (and also literally since the blood is life!) the killing of the animal (the taking away for life as a punishment for the sin) and the pouring of its precious lifeblood to cover the horrible pricy sins of humans by the priest under the authority of God:
Leviticus 17:11-12
“...the life of a creature is in the blood, and I have given it to you to make atonement for yourselves on the altar; it is the blood that makes atonement for one’s life. Therefore [this is why] I say to the Israelites, ‘None of you may eat blood, nor may any foreigner residing among you eat blood.’ ”
For these reasons, God reveals why he forbids consuming any kind of blood:
Because blood is the ‘life force’ of a living soul: that is blood is the fluid that keeps the rest of the body alive and functioning (this is covered in more detail in 'what is the breath of life?')! For this reason, the blood is ‘holy’ or ‘sacred’ and is not to be eatened and even when it drained it’s to be properly buried and it's so sacred that the shed blood is the only way God will truly forgive sins! Blood is that powerful!!!
If anyone did disobey God and consumed blood, then God says he would personally curse them and be against them and kill them (called here 'cut them off from the people': that is Israel)
God would be against them, and they would be banished or even possibly killed for eating sacred blood:
Leviticus 17:10-14
“I will set my face against any Israelite or any foreigner residing among them who eats blood, and I will cut them off from the people.”
This was so serious to God for a human to not eat blood that God himself would be against anyone in Israel or any Israelite bound by his covenant who ate blood and kill them!
Many years later, Moses reminded Israel when they finally conquered the promised land they could finally have meat whenever they wanted and craved it and could kill clean animals in order eat their meat, but he remined them when went they went to process the dead animal they were to drain out the blood ‘like pouring water out’ to keep from eating it with their meat so they didn’t consume blood and sin against God:
Deuteronomy 12:16-25
“When the Lord your God has enlarged your territory as he promised you, and you crave meat and say, ‘I would like some meat,’ then you may eat as much of it as you want….But you must not eat the blood; pour it out on the ground like water. Do not eat it, so that it may go well with you and your children after you, because you will be doing what is right in the eyes of the Lord.”
Moses also reminds them why to not eat blood inside of the meat: because the blood is the very life of a living creature! And should not eat the sacred life of a once living animal with the meat:
Deuteronomy 12:23
“…do not eat the blood, because the blood is the life, and you must not eat the life with the meat.”