"...not having any idea of where it came from."

Why would he care? What he's after are the factors in blood that improve performance that degrades as aging decreases those factors in blood, amongst other things. He doesn't actually even need whole blood, I believe, because plasma contains the parts he wants. I bet he can afford to tailor the factors he receives, to fine tune his anti-aging regime by dialing in just the blood factors he wants at specific levels he wants them at. These things can be centrifuged and otherwise filtered to be commingled from many sources into a custom aging retardation cocktail per his precise specifications.

It's not like he meets his victims/hosts, and cares who they are or what they want.