Blood magic is suddenly inserted into your current/most recent campaign world

I would use spells to enhance my blood and then learn blood magic the wreck all the people in town. Play jokes like sling blood down the town wenches leg and make it look like she has aggressive periods.

Already exists; vital force, the "soul of the body", is one of several sources of magical power, and blood is a convenient way in which to extract, store, and transport it. It's primarily used in medical and biological applications, in the construction of automata, and in a wide variety of morally questionable or unquestionably immoral situations.

I wish I could do Maria's blood bullshit on my settings

Yeah, already exists. Legality varies by nation, and by whether or not you're using willing subjects or taking enough to kill, but public sentiment is almost universally against it. This drives its use into the shadows of the world, which in turn increases the opportunities for abuse and misuse. It also has a large following among the more decadent nobility, bored socialites who get a thrill from dabbling in the dark arts.

I'll tell you how the party reacts when I find a group that can get its shit together long enough to play a game that lasts more than three sessions, and make characters with a backstory more involved than "I wanted an excuse to hit things with a sword." Not gonna waste the setting I've worked so hard to create on these yahoos.

I am playing 40k, pretty sure it doesn't make a difference

Blood for the blood god, etc etc.
youtube.com/watch?v=9-gSJW3sHXE

Horror, and sexual arousal respectively.

>Mass Effect RPG
So does it apply to all blood or human blood specifically?

Also, fuck.

It already is, though it's less "Using your vital essence to fuel dark rites" and more "controlling the blood of yourself and others".
If you want to sacrifice bits of yourself for power, that's fire magic's shtick.

Given that it's the Nanoha setting, the TSAB comes down hard on it, heavily regulates the practice and starts a research program to figure out the ill effects and to try and head off illicit blood smuggling programs to fuel any dangerous rites. Knowing the setting, a blood magic based Lost Logia (magical superweapon/potential cataclysm) is likely on the cards somewhere.

Oh yeah, forgot.
General populace's reaction would depend on province.
The more wary/religious ones would burn the witch, while some of the less superstitious or more pragmatic would react based on what end it was being used for.
Also, certain age groups in some provinces can have wildly varying reactions. The elders might freak, the kids might point and say "wow".

The party would take the wait-and-see approach.

It already is. Mostly it's a cheap way of empowering spells if you don't mind risking your health.

Her blood is literally magical though

I'd say it would actually cause Khorne to reconsider giving Sorcerers his mark if that happened.

>Metal Gear Rising: Revengeance (2013)

Already has it. There's a skill you can take that lets you keep casting even when you shouldn't be able to but you lose constitution (which is restored the next time you rest)

That was such a good game.

Yay! Another excuse for me to be a bastard.

It's already a thing. That's how you become a vampire

Given how magic is drawn taking blood from elder godly beings, Blood Magic seems fucking moderate in Dragon Age despite the Chantry's clampdown. Every form of magic in Dragon is blood magic.

In Skyrim, Alteration allows one to convert their lifeforce into magicka.

All magic in my setting is blood magic. Simple spells require a small amount; usually, a small cut will suffice. More complex rituals require sacrifice.

Adding on to this, when blood is consumed to fuel a spell, it turns to ash and disintegrates, leaving nothing behind. While it is possible for a mage to use magic without drawing blood, this could prove fatal if they're not extremely careful.

As for how people view magic, it's generally reviled and spat upon. The only practitioners are secluded witch covens, and those don't have the best of reputations. Because of its rarity in normal life, many people are ignorant of its existence.

Mild interest?

My party isn't a bunch of intensely insecure internet nerds who freak out at anything they could perceive as being appealing to the edgy demographic. It's like how closeted gay people act overly homophobic to compensate - Veeky Forums is way too intense about this shit. Who cares? Blood magic sounds neat.

I think I might give a bad guy Blood Release in my Naruto game. No idea how it'd go, though.

With a loose enough definition of both 'good' and 'game', yes.

Maybe. I'm just a shitter that doesn't pay attention but I have no idea what to call these satan circles that every magical fucker in art seems to have.

Considering I'm playing Double Cross, technically it already has it even if Renegade isn't exactly magic.

If actual blood magic appeared as a thing, the party would probably still just assume it's renegade bullshit and continue doing whatever they were doing at the time.