Plead the Blood?

[Pendragon]

I've heard the phrase 'plead the Blood' all my life, but never really thought much about it or studied it, and I've never used the phrase in prayer, because I never understood exactly what it meant - and still don't. Recently I started to wonder just exactly where in scripture is the inspiration taken to use this vague phrase in prayer. Every time I hear it used, it is in relation to a whole host of things ('I plead the Blood over [cars, house, finances, the President, Congress, children, etc.]'). It seems to me like it is a piece of tradition that maybe at one time was worded differently and possibly had a different meaning, but now has become almost an incantation of sorts with no biblical background.

I understand the principle of Jesus' sacrifice, and how he was the spotless lamb, and that his blood was shed in our stead. I just don't see anywhere in scripture where anyone pled the Blood over themselves, others, or inanimate objects or finances. I also don't see why you would need to plead for Jesus' blood to be applied in your life if you are already saved. Is His blood somehow less powerful in the life of a believer unless they plead for it regularly?

Doesn't make a whole lot of sense to me at this point, but if anyone has some scriptures or logic to explain this, I am open to understanding.