There is great power in the blood of Jesus Christ, which is applied to our lives when we walk in the light as He is in the light (see 1 John 1:7). But to simply recite the phrase 'I plead the blood of Jesus' is to reduce the power of the blood to something akin to witchcraft. There are no phrases or strings of words that have power in and of themselves. Even the words 'in the name of Jesus' have no power when they are simply repeated in a mechanical way. We may as well say 'hokus, pokus, dominokus.' The effect will be about the say. God responds to faith, not certain pseudo-spiritual incantations.