Original sin is a difficult doctrine.
You can read in detail here from the catechism:
The catechism, with my bolding below, says that it is a msytery that we know through revelation and faith, without fully understanding. It also says that it is not counted against each of us as a person fault, so to say God “blames” us is not quite correct.
404 How did the sin of Adam become the sin of all his descendants? The whole human race is in Adam “as one body of one man”.293 By this “unity of the human race” all men are implicated in Adam’s sin, as all are implicated in Christ’s justice. **Still, the transmission of original sin is a mystery that we cannot fully understand. **But we do know by Revelation that Adam had received original holiness and justice not for himself alone, but for all human nature. By yielding to the tempter, Adam and Eve committed a personal sin, but this sin affected the human nature that they would then transmit in a fallen state.294 It is a sin which will be transmitted by propagation to all mankind, that is, by the transmission of a human nature deprived of original holiness and justice. And that is why original sin is called “sin” only in an analogical sense: it is a sin “contracted” and not “committed” - a state and not an act.
405 Although it is proper to each individual,295 original sin does not have the character of a personal fault in any of Adam’s descendants. It is a deprivation of original holiness and justice, but human nature has not been totally corrupted: it is wounded in the natural powers proper to it, subject to ignorance, suffering and the dominion of death, and inclined to sin - an inclination to evil that is called concupiscence". Baptism, by imparting the life of Christ’s grace, erases original sin and turns a man back towards God, but the consequences for nature, weakened and inclined to evil, persist in man and summon him to spiritual battle.
The most important aspect of original sin is what God brings out of it. This is described here:
412 But why did God not prevent the first man from sinning? St. Leo the Great responds, "Christ’s inexpressible grace gave us blessings better than those the demon’s envy had taken away."307 And St. Thomas Aquinas wrote, “There is nothing to prevent human nature’s being raised up to something greater, even after sin; God permits evil in order to draw forth some greater good. Thus St. Paul says, ‘Where sin increased, grace abounded all the more’; and the Exsultet sings, ‘O happy fault,. . . which gained for us so great a Redeemer!’”
O happy fault!