How is man created from semen, which is made from blood and is impure?

Blood is not ruled to be impure until it exits and is shed. As for it being inside the animal's body and veins, it is not impure, because the original ruling concerning it is purity, and there is no evidence for its impurity in this state. Even if we assume that blood is impure while in its place, semen is pure because it has transformed and undergone a change from the original characteristics of blood to other characteristics. Impurity is removed by transformation and a change in description, and what matters is the current description of the thing, not its origin.