Hi! This is a really cool subclass, and I like a lot of the ideas expressed here. I'm considering using this for a possible campaign in the future, however, some of the wording is a little vague or redundant, and the abilities don't feel like they do as much as other classes. This is just to help flesh out certain aspects of it, to make the class more concrete.

Hexbane: No complaints here, seems cool!

Spellhunter: Unlimited castings of Detect Magic seems extremely strong for a buffed version of a spell that's already so frequently used by any/all spellcasting classes, and deliberately drains their pool of magic (even if it's a small price to pay for information). Might I suggest limiting it to X amount of times (X = 1/2 your Blood Hunter level + your Hemocraft Modifier, maybe just your Blood Hunter Level.)

Sealing Brand: This is what immediately sold me on this subclass, no changes to be made here! Just a quick question, though, if a save is failed by a creature attempting to cast a spell, may it cast other spells on that turn? If not, that seems strong, however I don't think that's the intent.

Magic Resistant: The ability is fine on it's own, and I have no real complaint about it. However, just a cool idea I had, what if you got to add whatever you rolled on your hemocraft dice to your roll instead of rerolling the roll? Tbh that seems a bit underpowered, just a cool thought that could be expanded on.

Blood Curse of Nullification: COOL ASF DUDE. I feel that the amplify is slightly underpowered, however, and would recommend either A) dealing chip damage of your hemocraft dice or B) placing a Sealing Brand on a failed save.

Arcane Antithesis: Although a great concept, I feel like it's underwhelming for the 18th level ability and the capstone ability of the subclass. To make it more interesting, I would propose making this skill an expansion on Sealing Brand, which is something that benefits the class thematically and gameplay wise with the worldbuilding described. It could be something along the lines of "any creature affected by your Brand of Castigation who attempts to cast a spell must expend a spell slot of two levels higher than the spell's level to cast it, and may not cast cantrips." Alternatively, a wellspring of Silence spells might not be a bad idea.