In any case, it probably involves making holes, as McCoy thinks drawing blood without leaving holes should be impossible in "Obsession"...
Might be the standard hypospray (which is used for the bloodsucking in, say, "The Adversary") simply delivers a numbing agent that it subsequently also withdraws, taking lessons from the common gnat. Might be the hole in question is made using special future techniques that close it afterwards (after all, dermal regeneration is an art form well mastered in Star Trek).
Might also be 24th century devices use the transporter effect and would make McCoy gasp like he did in "Obsession".
Timo Saloniemi