Black heart of potato is the result of

The black heart of potatoes is the result of High temperature during transit, Poor ventilation in the store, and High temperature of soil during growth and maturation of tubers in the crop field.
The black heart occurs primarily in storage when the tubers do not receive enough oxygen. Blackening of the tuber center follows acute oxygen deficiency associated with either low temperature in confined storage or high field soil temperatures. The tissue dies from the inside out and turns jet black.