He has no special equipment. His time with the body is limited. What's his best option for getting something he can use for a blood type?
I should add that he also knows that a competent medical examiner will be aware that the scratch (or whatever he does) was done long after death, and he might get into trouble for this. If there's a more subtle or less invasive way to do what he needs to do, he'd prefer that. But he'll take what he can get.