Is drinking blood Evil?
Even in the world of "objective morality" created by D&D alignment, drinking blood isn't necessarily Evil. Why? Animals drink blood. To them, it's just basic sustenance, no different from eating meat.
Once you attach a metaphysical component to the act, though — I am drinking your blood in order to steal your courage, for example, or feed on your very soul — then it rapidly becomes Evil.
These are the two basic poles. Everything in between, such as whether non-magical cannibalism between sentient beings is Evil, is pretty much up to the group. Generally, I'd go with this: if you think desecrating a dead body (not the spirit) is Evil, then most forms of blood-drinking should be as well; if you think desecrating a dead body is no different from breaking a lamp, then most forms of blood-drinking should be as well. Either way, it's not something to keep secret; get group buy-in about what the one right answer is, and proceed from there.
Is Blood Transcription Evil?
In the case of Blood Transcription, the answer is provided for you: the spell has the [Evil] tag, which is described as:
Evil: Spells that draw upon evil powers or conjure creatures from
evil-aligned planes or with the evil subtype should have the evil
So, the game is telling you that Blood Transcription is innately Evil. There's two ways to interpret this:
- Blood Transcription involves performing an action that's inherently spiritually violating, so it draws on Evil power to accomplish its effects.
- Blood Transcription is a spell based on Evil power, so it accomplishes its effect using some sort of Evil method (likely spiritual violation) .
Either way, using Blood Transcription spell is an act of Evil. The biggest difference is really whether you could create an analogous non-Evil spell using some alternate arcane force.
Note that merely committing an Evil act does not cause an alignment shift. Part of being Neutral is, as you said, the willingness to occasionally do Evil. In the world of D&D alignment, character committing "justified" Evil acts are still doing Evil. That's what supernatural objective morality is all about.