The reason that white sweet potatoes turn black as you peel them is due to oxidation of the inside of the white sweet potato.
When you remove the skin from the white sweet potato the inside of the white sweet potato is then exposed to the air and oxygen which causes oxidization.
The longer you leave the white sweet potatoes out the more the white sweet potatoes oxidize.
That goes for other potatoes as well that you leave exposed to the air with the skin removed.
There's nothing harmful about the oxidization of the white sweet potatoes or any other potatoes that turn black or just turning a different color.
The potato might not be very good tasting but it's still safe to eat the potatoes.