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
descriptor.
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.
There's no such thing as senseless violence, according to the one who commits it. Characters who kill or torture without at least an internal justification are crazy, not evil. You don't have a reason to kill people in the party or at random, so you don't. This doesn't make you nonevil.
Also remember that just because you're Evil doesn't mean you're a villain. Many Evil characters have no ambitions higher than their own survival and/or comfort; they don't aspire to great power, nor to purge the world of the target of their hate. They're just trying to get by, not so different from the rest of us.
The traditional list of Seven Deadly Sins was originally compiled not so much as a list of inherent sins, but a list of reasons that people sin. This makes it a great source of motives for Evil characters. I prefer to expand the list to nine, adding Fear as Wrath's twin in the fight-or-flight reflex, and Despair because it was actually in the original list; Sloth replaced it later.
Avarice: The key to happiness is having things. I will have it all.
Envy: I deserve it, not them. I will have it from them.
Gluttony: Pity those not at the top of the food chain. I will never be in that position.
Lust: I want to do it all, and I will let nothing get in my way.
Pride: I must be better then them: so much so that my superiority is never even questioned.
Sloth: I just don't want to do it. Let them do it for me. If they refuse, make them do it for me.
Wrath: They will never hurt me again. I will punish them for what they did, and leave them unable to do it to anyone else.
Fear: They must not be allowed to hurt me. (Note the lack of an again here: this is one of the big differentiators between Fear and Wrath, but it can make a huge difference in the character).
Despair: I just want the pain to end. Giving it to others helps.
Also keep in mind that these are core motivations. Any one of them will need to be elaborated upon. What is it? Who are they? How does the character plan to achieve this goal? Also worth noting is the lengths that your character goes to to hide her motives. Evil characters often prey upon one another's weaknesses, and while Wrath-type characters might not worry about seeming weak due to their motives, Sloth-type and Fear-type characters likely would. These folks are likely to construct a facade, often but not always based on Wrath, as a matter of posturing.
Your character sounds like a Wrath-type, with a focus on the undead. Because her main focus is on something that is not so amenable to the survival of humanity in general, she can get along decently well in society, and even be a very useful sort of person to have around. Some might even mistake her for heroic. But she has a twisted fight-or-flight reflex: any slight or injury, real or imagined, runs the risk of touching on that trauma, for reasons that make sense only to your character (if they even make sense to her). She might lash out disproportionately at small threats, or even against things she mistakenly believes to be threats, but are not.
Best Answer
The GM can refuse an alignment shift if the player is exploiting their character's alignment.
I looked through the GameMastery Guide chapter regarding player character alignment to see if there is any precedent on this. In short, it acknowledges that the traditional 3-by-3 alignment system is overly simplistic and can be interpreted in various ways. The GM should arbitrate and communicate with the players when dealing with morally gray cases.
It sounds like your player is trying to achieve a temporary alignment shift, with this logic: If their evil character does some good deeds, then their alignment will shift to good, and therefore they won't ping as evil for Detect Evil. After that, they can return to their evil ways, at no cost.
The following excerpts in the "Changing Alignment" section are relevant:
This means that both the player and GM should agree before an alignment change takes place. An evil creature can prompt an alignment shift by consistently doing good, but no alignment shift actually occurs if the GM disallows it. And the GM may disagree because...
The character is already evil, and intends to continue doing evil things in the future. The GM can decide that their attempts to score some cheap karma points are insufficient to change their character's alignment.
As GM, you should probably communicate your interpretations on alignment to your players, and explain this beforehand. It may seem unfair if they believe their character is non-evil and thus safe from the paladin, and suddenly the GM springs a "Surprise! You're evil!" on them.
Side note
Your player seems to be under a faulty premise about what Detect Evil does.
It does not detect the presence of good. If evil is in the area being analyzed, then the presence of good cannot trick the spell. Unless the good deeds were enough to shift the creature's alignment to non-evil, any good deeds are irrelevant for the purposes of this spell. The spell does what it says it does, and cannot be "fooled", unless one uses a spell or feature that explicitly says so.