It's not that it can't work

it's just really, REALLY combo-dependent. If you can't get the right cards (blood knight and at LEAST one divine shield to absorb), it doesn't work. Plus, even if you have a divine shield out there, it can easily be removed by just about anything. I have occasionally played around with blood knights in arena, as a kind of Sunwalker counter, and though it feels really nice, it usually isn't in your hand when you need it. The biggest problem of blood knights? They don't have charge, meaning you're unlikely to be able to use that power bomb unless your opponent's board is significantly weak, and/or they don't have a power card in hand like SW-Death, BGH (if 2+ shields), or just a silence owl.