So first I'm gonna explain how I think knowledge works with dark moon magic and white magic
White magic comes from all celestial bodies emitting light, that includes the stars, the moon, and the sun.
White magic provides light and healing, but it is not technically the source of truth.
Dark moon magic is mysterious and treacherous by nature, it's darkness leaving too much room for lies and deceit. However, it holds knowledge as well. White magic brings the truth of that knowledge to light.
Now how this relates to sage and fount, is that he is a part of the other realm. Light is the method of truth. stars guide through the dark of the night and lamps/sources of light give you the ability to see when in the dark. To see the truth, you must have light to navigate the darkness of infinite knowledge.
And knowing things doesn't necessarily mean those things are true.
Partially why I believe Fount has a star as the shape of his light, and a ball of light as a method to guide him.
When Fount corrupted, the light that he used to guide himself and others were completely eradicated. To represent he wouldn't be a guide, the stars were replaced by eyes. The ball of light was discarded, and the moon reflected that he had become completely immersed in darkness and deceit.
Within the context that it is confirmed that sage is the only one to not fall out of every Fount, it is within my personal belief that the reason for such is him becoming the beacon of truth, not allowing that natural urge of the other realm to corrupt him.
Sage, tho now representing absolute truth, is still a being of knowledge, and is still a part of the other-realm. His magic is naturally darker than the pure white of white magic because of this.
(I'm gonna have to add on to this because I can't add more images @cheesecakemermaid1048 here this is for now)