The best I know how to describe it, we are sinners in three aspects: sinners by representation, sinners by nature, and sinners by practice. Only the last of the three deals with sin as we commonly think of it. The first two are part of the curse brought on us by the fall of Adam in the Garden of Eden.
Sinners by Representative:
In Eden, Adam was the great champion of the human race. In Hebrews 7:9-10, Levi is said to have been in the loins of Abraham and paid tithes to Melchisidek even though he had not yet been born. In a similar way, I was in Adam when he disobeyed God and partook of the forbidden fruit. What he lost by that sin, I lost. He lost paradise on earth. I lost it. He lost eternal life and so did I. That is why Paul states that "in Adam all die" (1 Corinthians 15:22). Romans 5:12 states further, "Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned." "For if through the offence of one many be dead" (Romans 5:15) refers to the effects of Adam's sin on all. In 1Samuel 17, Goliath is the champion for the Philistines. He fought their battle for them. Therefore, "when the Philistines saw their champion was dead, they fled" (1 Samuel 17:51). When he lost, they lost. In like manner, when Adam sinned, we all suffered the consequences.
Sinners by Nature:
Something else happened when Adam sinned. His very nature was corrupted and that corrupted nature is passed on to all of his descendants. Adam was created in the image and likeness of God (Genesis 1:26-27). However, after the fall, Adam "begat a son in his own likeness, after his image; and called his name Seth" (Genesis 5:3). Although we retain a shadow of the image of God in ourselves, our primary nature is the fallen nature of Adam. In the New Testament, the word "flesh" is often used to refer to this fallen nature (although flesh also refers to the material part of man and the meaty portion of the body according to context). Paul is referring to this sinful nature when he states, "For I know that in me (that is, in my flesh,) dwelleth no good thing" (Romans 7:18). Jesus came "in the likeness of sinful flesh" (Romans 8:3). The wording here is important. Naturally, the flesh referred to here is sinful. But in the case of Jesus, it was not sinful though it was visibly no different than all other flesh.
Sinners by Practice:
This refers to our practice of committing actual sins. There is no need to dwell here. However, I want to make an important point. It is this practice of sin that actually commits us to judgment in hell. The fall of Adam as our representative head condemned us to live in a fallen world. The corruption of Adam's nature by sin caused all of his descendants to be born with a corrupted nature that tends to sin and fails in many ways. But it is only our actual practice of sinning that condemns us to an eternal hell. This explains why infants who die do not go to hell.
What About the Blood?
You mentioned the teaching of sin being in the blood. I would disagree that sin or even the sin nature per se is in the blood. In fact, the blood of infants is called in Jeremiah 19:4 the "blood of innocents." If blood carries the guilt of sin from birth, then the blood of infants would not be innocent blood. It is true, though, that blood carries the guilt of those who shed innocent blood. "Thou art become guilty in thy blood that thou hast shed" (Ezekiel 22:4; see also Genesis 9:5-6; Ezekiel 22:1-4, 13).
On the other hand, the scriptures do seem to teach that blood is a carrier of corruption. Though Christ in His resurrected body had flesh and bones (Luke 24:39), "flesh and blood cannot inherit the kingdom of God" (1 Corinthians 15:50). By comparing these two verses, we can see that the offending item is the blood. From a practical point, an embalmer quickly removes the blood from a dead body because it corrupts so quickly. In the Bible (Exodus 7:17-18; Revelation 11:6), the changing of water into blood was a corrupting plague. The blood is seen as something corrupt and unclean. The same could be said of how blood in treated in the law.
I know of no direct statement in scripture teaching that the sin nature is transmitted by the blood. However, many believe that it is. This is not to say that the sin nature is in the blood exclusively as something that dwells only in the blood. The AIDS virus affects the entire body, but can be transmitted to others through the blood. I think this is the concept most people have when they talk about the sin nature being transmitted through the blood.
This would explain some of the things that are said of the blood of Christ. His blood is called "the innocent blood" by Judas (Matthew 27:4). Although this phrase is found in other places in the Bible, it seems to be used in a special way here. The blood of Jesus was not just innocent; it was "the" innocent blood. Acts 20:28 tells us that God purchased the church "with his own blood." That is, it was the blood of God that paid for our sins. 1 Peter 1:18-19 tells us that we were not redeemed with corruptible things but with the "precious blood of Christ." Therefore, the blood of Christ was not corruptible. It was evidently divine blood.
This is how Jesus could be born in the "likeness of sinful flesh" (Romans 8:3), but without the sin nature. His flesh came from Adam, but His blood came from God. He experienced all the weaknesses of the physical body, but He did not have the sin nature. He came as the "last Adam" (1 Corinthians 15:45) and the "second man" (1 Corinthians 15:47). He came to win where Adam had lost.
I know there is a lot to digest here. I hope it has been a help to you. But through it all, we must remember that Jesus Christ came to give us the victory over sin and all its consequences. Because we are sinners by representation, we are subject to the presence of sin in the world. Because we are sinners by nature, we are subject to the power of sin in our lives. Because we are sinners by practice, we are subject to the penalty of sin. But Christ has redeemed us from all three: from the penalty of sin when we were saved; from the power of sin as we live for Him; and, from the presence of sin when we go to be with Him. Praise the Lord for His goodness!