I can tell you the exact reason why this show failed to keep me invested: The human that inhabit this world are truly unlikable. All this anime had to do was show me just one of the girls turning into a huge monster and I would understand there prejudice.
Let’s talk about “elfen lied” for a moment. In this show we see human mistreat these girls with horns in vial and disgusting ways (allegory: humans are capable of being just as brutal) however, we see the girls create fear within humans, we see them go on killing sprees and we see how that emotional affects them (they have no control).
The problem with Black Bullet is that the human brutality (beating a cursed blind girl to the brink of death) doesn’t level out with the actions of these girls. The only girl that gives these human any need to worry is Kagetane Hiruko partner and she is still obedient even in her deep murderess trans.
The ending to this anime was needlessly brutal and made very little sense in the long run.