Hail comes with a thunderstorm. The lightning in the thunderstorm can ignite a forest fire, a grass fire, or perhaps even a house fire. The hail can also destroy crops, as it did in the seventh of God’s plagues upon Egypt (Exodus 9:24–25). Both the destruction of crops as well as the fires can result in loss of life; hence the reference to “mixed with blood.”
7 The first angel blew his trumpet, and there followed hail and fire, mixed with blood, and these were thrown upon the earth. And a third of the earth was burned up, and a third of the trees were burned up, and all green grass was burned up.