The blood of Jezreel refers to the bloodshed committed at Jezreel, including the murder associated with Ahab and the violent judgment executed by Jehu against Ahab’s house.
Hosea uses the expression in announcing that God would reckon with the bloodshed of Jezreel. The references connect it with the killing of the Lord’s prophets and with the judgment on Ahab’s house.
The phrase recalls the events surrounding Jezreel, especially the violence involving Ahab, Naboth, Jezebel, and Jehu.