I'm not gonna say much, all I have to say is that questions like this is not meant to be answered by all Christians, most people who identify themselves as Christians have little to no connection with god, by connection I mean contact, I myself identify myself as a Christian, but I'm in no way saying I'm better than any other Christian out there.
Like many people mentioned already, blood is life and life comes from God, so idk maybe he sees it as the most important thing we can give him. The fact is we shouldn't really question God, he's the creator and maker of everything, he knows what's best for us because he has seen our future.
The bible said in _Isaiah 55:8-9_
For my thoughts are not your thoughts neither are your ways my ways, declares the LORD. For as the heavens are higher than the earth, so are my ways higher than your ways and my thoughts than yours.