All we really know is that the blood could reverse certain types of damage such as radiation poisoning, and only if administered reasonably soon after clinical death occurs (remember, McCoy ordered Kirk's body placed in stasis as soon as he realized the possibilities). That's not entirely unreasonable. It's already possible to revive people who've been clinically dead for several minutes. And we know that Trek medical science is even more advanced. It tends to be forgotten that the first season of TNG portrayed it as advanced enough to reverse recent death in multiple cases, such as Yareena in "Code of Honor" and the cryosleep subjects in "The Neutral Zone." This was largely overlooked in later seasons, though.