Today I cut open my potato and found that the middle was black. What cause this to happen and are they still consumable?

This is called “Hollow Heart” and is a common problem caused by alternating periods of slow and fast growth during the months after the potatoes are planted. It is very hard to detect in grading and sorting as there is no external indication of anything wrong with the potato. It is not a disease, while unappetizing, the affected area can be cut out and the potato consumed.

Read more at Gardening Know How - Potato Hollow Heart: What To Do For Hollow Heart Disease In Potatoes