Bloodbending is less about using blood to do things, and more about controlling people by bending the water in their blood.

[When she talks about the magic that the Alchemist wants to generate, feeling that frisson of fear slide down his spine...and he wraps his arms around himself a bit, shoulders hunching]

A metalbender can't use bloodbending, but if it's like what you're talking about with those witches... [he shudders] It's really terrifying.

[he's maybe a little paler. like girl he is super scared]