[quote=@Jouska]
Would you have something against a character whose ability would be to control his own blood (for example enabling him to soldify it into a shield or weapon)?
[/quote]

As long as you limit it correctly it's fine. The human body needs a certain amount of blood to function. Using it as a weapon/shield is dangerous... If you're familiar with the anime Deadman Wonderland you'll know what I'm talking about.