Currently you don't have much reason to want to take a PC's blood - unless you're a hematologist or a Nephezar.
I think it would be interesting if you someone's blood sample to lower the DCs of certain spells. Blood could be used as consumable to scry them easier, or to place a curse on them at a lower dc. It gives PCs more incentive to guard their precious bodily fluids and for the nefarious to want to take them.
I'd love to see more ways to inflict medium term consequences for a PVP defeat. So if you subdue someone, you can have a massive advantage the next time you fight them. As that discourages coming back fullbuffed against someone who beat you previously, as they have blood/a voodoo doll of you/whatever. I thought about suggesting this, but the problem is that even if you make blood be consumed each time you do this, the solution is to yank a lot of blood at once. So it's not a serious limit.
I think a better way of doing it would be to sell "Curse Dolls" or something. Items that you can use on an unresisting/subdued PC. It's then named after them, and you can use the item's special power to apply a dispel effect. Or anything else that's a bit more useful for defence than surprise attacks. This way you pay maybe 1-200 gp up front for the doll, and weight becomes a factor if you stockpile loads.
That way, it's accessible to non-magic PCs, and sparing an enemy doesn't mean letting them plot revenge easily.