The original story, The Little Mermaid, is by Hans Christian Andersen. It's based on Danish folklore. Andersen described Ariel as having blue eyes and white skin (I can't find any info on her hair color). Here's the story.

Passage describing her skin color:

When something like a black cloud passed between her and them, she knew that it was either a whale swimming over her head, or a ship full of human beings, who never imagined that a pretty little mermaid was standing beneath them, holding out her white hands towards the keel of their ship.

Passage describing her eye color:

They were six beautiful children; but the youngest was the prettiest of them all; her skin was as clear and delicate as a rose-leaf, and her eyes as blue as the deepest sea; but, like all the others, she had no feet, and her body ended in a fish’s tail.

There are lots of myths and folklore about sea creatures and mermaids in the Middle East, Africa, and India. Disney could've pulled from any of them. They're not. They're intentionally changing a white character to black.