As the novel develops, it is Jack's hunting that brings the motif of blood to the fore of the boys' imagination, and therefore the reader's. As they begin to kill pigs and delight in the violence and the blood more and more, they begin to turn from civilised boys into bloodthirsty savages. Note how these two states are contrasted explicitly in the following quote from Chapter 4, which is when Jack has just killed a pig, leaving the fire to go out:
Jack stood up as he said this, the bloodied knife in his hand. The two boys faced each other. There was the brilliant world of hunting, tactics, fierce exhilaration, skill; and there was the world of longing and baffled common-sense. Jack transferred the knife to his left hand and smudged blood over his forehead as he pushed down the plastered hair.
Blood here is something that therefore directly divides Jack in his state of savagery from Ralph, who is still clinging on to civilisation and innocence at this stage in the novel. Of course, as the novel develops and the hunters move on to bigger prey, their loss of innocence becomes more profound and staggering.
A good statement you could use to describe this therefore might be: The motif of blood and the boys' obvious fascination with it charts their journey from innocence to bloodthirsty savagery.