The things people typically have phobias of, it is evolutionary. Heights, spiders, snakes... things that were a danger to us (and still can be.)
Interesting thing about blood. I know a few people who have blood phobias. Some aren't phobic of other people's blood, but will pass out seeing their own. But an interesting thing about blood, the color red makes us alert. That's why it's used as a "warning" color. I learned about this when I was road tripping a lot and needed to find tricks to stay awake. When you're drowsy or not feeling attentive, the color red can make you more alert because it triggers that part of your brain that says "DANGER."