They wouldn't change color. If you choke a smurf,you would restrain the oxygen from turning the blood red, which creates the blue color that humans turn. Since smurfs are already blue from god knows what, it would either not change at all or turn into a darker shade of blue.
Hemoglobin that contains ox is called oxyhaemoglobin and turns a bright red, but once it lets out the ox it picks up carbon and becomes carboxyhaemoglobin and a dark red/purple color.
No. It's never blue. It turns really dark red but it looks blue to us. And no, when blood is revealed to oxygen it turns red, which is what you see when you accidentally cut yours, but when you don't have any oxygen, your blood turns that dark red (blue) color.