Sigh. My answer to it is here, but I thought I'd supplment it with an actual photograph, taken with my cell phone, of some blood I recently had to give to a patient. The blood was originally drawn from the venous pool of another patient (obviously) and its seal in the bag ensures that it can't get reoxygenated. And the colour of this blood, of course, isn't blue. It's a deep maroon.
Told you so.