"blood hunter suffers the same problem all of mercer's homebrew does
[...]
blood hunter basically punishes the player (HP reduction, loss of control in 1 subclass) for wanting to be what amounts to a specialized ranger in flavor. the subclasses are not balanced well between each other and are filled with a lot of weird incidental bonuses that 5e largely moved away from on purpose or the hack job that is like 1/3rd pact magic.
it's workable in that numerically it isn't going to break the game, but it's not made in a mindset that is compatible with the conventions of the system. it's a hack…