[RPG] Can a Half-Dragon’s alignment change from their dragon’s alignment type

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:

1. Blood Transcription involves performing an action that's inherently spiritually violating, so it draws on Evil power to accomplish its effects.
2. 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.

Alignment Usually Isn't Hard To Determine

Alignment often isn't that hard to figure out anyway. Spells that can detect it are readily available, some classes (like Paladins) can detect evil as much as they want, and in certain cases you can discern it from their actions (a Cleric casting a Good spell can't be evil, because a Good type spell can't be cast by Evil Clerics).

It's possible to take steps to conceal it, but in my experience not many people do that.

So I don't think the real risk here is that you're going to make alignment too easy to determine.

Overemphasizing Alignment

To me, the biggest concern is that by having the world reflect alignment so readily, you're making it more important. Being Good is so important in your world that all magic is affected by it, even when the spell is thematically neutral and doesn't do anything that relates to alignment.

You've elevated alignment to be a more prominent thing. Considering how rigid the alignment system can be and the problems it can cause with player behavior, that's not something I would want to do.

Consider what that means? Normally if I see someone cast a Fireball to take out some creatures, I'd just determine what to think about that based on what I saw, and if I cared about those creatures or not. But now I'm seeing an Evil Fireball. If I'm a Good character, do I have to do something about the evil spellcaster? What if they're on my side? What if the people they're attacking are also evil, but those people are using weapons so I can't tell?

The alignment is so in my face that it distracts from trying to figure out the motivations of the people involved, because literally the first thing you've told me is "that Wizard is evil."