The blackheart is a humanoid brood parasite. Adult blackhearts will sneak into the home of a married couple, with at least one child, during the middle of the night. Using a long needle-like finger, the blackheart will inject a tiny egg into the sleeping human woman's abdomen. The blackheart is an extremely stealthy creature, and will do all this without making a sound. Over the next 8 to 9 months, the woman will experience what seems to be a normal pregnancy, and she will give birth to a baby which appears to be a normal human. But in reality, it is a blackheart.

Baby blackhearts retain all the knowledge and memories of their blackheart parent. The baby will play the part of a human infant to fool the parents into thinking it's their own child. In order to grow, the blackheart slowly siphons off the life energy of their human false siblings. The human children in the blackheart occupied household will gradually fall ill as the blackheart steals their energy over the years. If the children die, the blackheart will begin taking the energy of the human parents, although it prefers not to do this until late into its development, as it needs to parents to be well so they can look after it. Once the blackheart has taken enough energy, it will finally transform into its adult stage and leave to find new victims.