I think what is going on is the running water is removing the starch from the outer surface of the potato.
When this happens as the potato dries it wont turn black.
One of the things I do when I cut up fresh potatoes is to put them in water and use what we call fruit fresh in it.
This is mostly ascorbic acid and citric acid.
The black ones I made I tried and then tossed.
They tasted good but was creeped out by the color and was afraid I would get sick.
Worth