Blood spots are a sign that the egg was fertilized. Instead of being a bad sign, it is a good sign.
Bad eggs will look cloudy and curdled, or will smell. Also, never use an egg from the carton that has a visible crack.
I grew up on a farm. The eggs were dirty, so we washed them. But we always kept them in the fridge and ate them within a few days so the risk was very small. The fresher the egg the less likely it will be bad.