Is the blood in the egg or on it? Blood spots in eggs have a bit of heredity in them, some hens are more prone to them by their genetics. They signify that a blood vessel broke in the oviduct while an egg was passing through, before the shell gland, it usually is not significant.
Rough handling, being chased or stress can cause blood in eggs too by causing vessels to break.
Blood on the outside of the egg can mean that a vessel broke near the end of the oviduct or it can mean the hen has a torn or pecked vent. Pick her up and see if you can see blood around the vent and if so she needs to be moved away from the other hens so they don't peck at her.