If you're planning to get the Weak Blood Shards from the mobs of your mobfarm, dont kill the mobs if they are having only 0,5 hearts. The chances are way lower if they have low health. Just cut off the dropshaft with pistons or so an kill the mobs without them getting falldamage.
even better: get 1 demon blood shard, and automate the production of imbued slates>Shapeless crafting 1 Blood shard+1 Imbued Slate+ Master Blood Orb = 5 Blood shards = GG Always did it like that