The word “blood” appears in the novel 28 times. While the meaning of the word does not change as the novel progresses, it does mean different things depending on the character we are focusing on. For example, when we are reading the scenes involving Alim Nassar, blood is associated with brotherhood (it also seems to have a connection with the gang that he is the leader of, although I don’t think that it is meant to be a reference to the Bloods, which I believe started in LA around this same time). On page 73 it says “[Alim’s] brother (mother’s son, not just blood) waited for him in a car that wasn’t hot.” This marks the important distinction that Alim makes in his mind between the literal meaning of blood and blood as a figurative indication of unity.
Another set of characters who attract the idea of blood, is Fred Lauren and Eric Larsen. When Fred Lauren kills Colleen, there is a lot of blood, and Eric is the one to go investigate the scene. Here we see the word blood with its most literal meaning, there is no other way to see this scene than as a violent act which spilled human blood. Each time Eric Larsen thinks of what Lauren has done, he gets the smell of copper stuck in his nostrils, because that is what the crime scene smelled like from all of the blood. It is this scent and this image stuck in Larsen’s brain that drives him to want to kill Lauren so badly.
For the rest of the characters “blood” is the embodiment of the disaster. The first mention of the word is not until page 72 of the novel, in the figurative sense that Nassar uses it. After the comet strikes though, it becomes much more prevalent in the narrative. Each time it appears you get more of the sense of loss and destruction, for example on page 239: “There was a baby in it. It didn’t move. Tim felt for a heartbeat, and his hand came out covered with blood. It was bright red, copper blood, and the smell filled the car despite the warm salt smell of the rain.” I think most of us were disturbed by this image of a poor, helpless baby dead in this disaster. Yes logically, any baby’s chance in this situation is slim to none, but the image is still terrible and furthers the sense of destruction.