Why pray the blood of Jesus over things? Because according to Revelation 12:11, the blood of Jesus is how we defeat the devil in the last days.

Revelation 12:11 says:
11 And they have conquered him [Satan] by the blood of the Lamb and by the word of their testimony, for they loved not their lives even unto death.

I once mistakenly interpreted Revelation 12:11 to mean that the saints would showcase the blood of Jesus by sharing their testimony, but not so! It turns out that the devil who was defeated two thousand years ago by the blood of Jesus, will suffer a second defeat by that same method.