Bad blood is a feeling of ill will, anger, or hostility between people. When there's bad blood, people aren't getting along.
Besides its literal meaning, the word blood has long been used to refer to people's emotions. Bad blood is a term for emotions that are negative. There is usually bad blood between a husband and wife who get divorced. During the American Civil War, there was a lot of bad blood between the North and South. If a friend forgets your birthday, there could be bad blood for a while. Bad blood tends to make people hot-blooded: angry.