Right on!
Scripture also doesn't tell us that Cain and Abel knew it requires blood for the forgiveness of sin, Christ's blood even symbolically by the blood of animals. Cain used his own work for forgiveness of sin and that didn't work, Abel obeyed. God is eternal and His ways are eternal.
When I look at the story of Adam and Eve God teaches Adam and Eve about Salvation through the killing of a sacrifice.
Gen 3:21 Also for Adam and his wife the LORD God made tunics of skin, and clothed them.
By killing the sacrifice he made tunics of skin and clothed their nakedness Just as Jesus covers our sin.
They understood that this was a covering for their sin. And that it pointed to the "seed" i.e Jesus.