The blood crucible is an item intended to work in tandem with the Altar of Blood. The blood crucible is crafted with netherrack similarly to a cauldron, with the exception of an ender pearl in the center tile. It will store up to 100 blood for you, gathered by collecting said blood in your essence keepers and vessels after you spilt it with multiple powerful blows from your glaive.
Destroying a filled crucible of blood will spawn multiple hemoglobin, which behave identically to slimes. You may right-click a Hemoglobin or Ectoplasm with an empty essence keeper/vessel to kill or divide it instantly. This also refills the empty essence keeper/vessel depending on the size of the slime (up to how much you originally had in the crucible).
Blood Crucibles (left and right filled with blood, middle empty)