Yahweh refers to Jerusalem as the city of blood. This refers to the murders and violence that happen in Jerusalem. (See: [[rc://en/ta/man/translate/figs-metonymy]])
The word “her” refers to Jerusalem. Cities were often thought of as a woman. This continues through verse 32. (See [[rc://en/ta/man/translate/figs-gendernotations]] and [[rc://en/ta/man/translate/figs-personification]]).