If I were the DM I'd say a Level 17-ish or higher caster might be capable of manipulating a target's blood. And with restrictions, such as him actually being a quite ruthless character.

Otherwise it's just a bragging player to be honest.

Remember that creativity is fun at first, but it is prone to eventually get one-patterned and dull, and the other players might even get sick of it. The various ideas presented by the thread creator are good because while each are useful, they're all somewhat situational.

This idea of manipulating blood is NOT situational. It will soon grow to be boring and thus it's a bad idea for the game. If a particular "at first unique" method proves too effective in various situations, chances are it wasn't a good idea to allow it in the first place. No matter how logical an argument is, it's a worthless argument if doesn't make the game more fun.