It's not overlooked in science fiction.Typically, alien race are treated as completely different beings. Because they are. To clarify, moving blood from one human to another can fail if there's a single protein on the blood cells from the donor that the recipient doesn't have.
An alien race presumably has a lot more difference than a single protein on a blood cell, and when I say presumably, I mean by definition, there's not really a way for such a being to naturally evolve like that. It's actually common in sci-fi to have newcomers to an alien world being unable to eat any of the lifeforms there.
To answer your Spock question, Star Trek uses soft science fiction, considering that Spock is half Vulcan, the other half being human. Meaning that Star Trek has decreed that (somehow) Vulcans and Humans have a similar enough genetic code such that they can produce offspring. (I think Spock might not even be sterile, though my knowledge of Star Trek doesn't extend to that.) That still doesn't mean that he could accept pure human blood, though, because it depends on how the internal biology of his systems work. And, again, since Star Trek is soft sci-fi, they don't have to worry about that. Honestly, all they have to do is go to the food synthesizer and order a cup of warm half-Vulcan half-human blood.