When Roger is attacked in a truck by a female zombie, Peter comes to the rescue, shooting her in the back of the head, spreading her face and blood all over Rog. In the next shot, he looks shocked at the blood all over him, but in another shot, he has no blood on his face at all.
I am pretty sure he wipes it off with the towel thing or something that he uses to cut off the circulation to his leg after he is bitten...i am pretty sure...