Black Inky Potato Core

Everything was fine while I was peeling this potato, but when I cut it, I saw this black inky patch in the core of the potato. There was no change in smell or texture of the potato, but I dared not eat it.

A friend suggested oxidation as the cause, but if there were true, why is it that the periphery of the potato was not oxidised?

Another possibility is an infection by potato blackleg, a seed tuber-borne disease caused by Erwinia carotovora subspecies atroseptica. However, the literature mentions clear delineation between healthy and diseased tissue. This lesion doesn’t look like it’s clearly demarcated.

Anybody who knows the answer, please let me know.