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Throughout the Dune series, 'blue within blue with blue' eyes (indicating both the pupils and whites of the eyes) are the mark of someone who consumes large quantities of spice.
This cause and effect is well known, but the reason for it is less clear. Is there any further explanation for why spice causes human eyes to become completely blue?
It seems that is done by tinting the sclera, cornea and iris of the eyes.
Extensive use of the drug tints the sclera, cornea and iris of the user to a dark shade of blue, called "blue-in-blue" or "the Eyes of Ibad," which is something of a source of pride among the Fremen and a symbol of their tribal bond
It seems that it can do so since, with high dosages, the drug is a mutagen.
TL;DR: The melange itself is blue. The eye tissue changes color by being saturated with it.
We know that continued use of the substance is required to make the eyes change, so it would appear to be a cumulative effect upon the tissues. Spice is generally thought of as a cinnamon-like substance, probably because the odor is constantly described that way. To make matters more confusing, it was depicted as a reddish-brown powder in both live-action adaptations, as well as the popular video games based on those adaptations.
In "God Emperor of Dune", however, it is described as being more like ground crystal - like sand itself, in fact. As for color, it is a deep blue that refracts light:
The place had filled Moneo with awe. Great bins of melange lay all around in a gigantic room cut from native rock and illuminated by glowglobes of an ancient design with arabesques of metal scrollwork upon them. The spice had glowed radiant blue in the dim silver light. And the smell-bitter cinnamon, unmistakable.
The color is mentioned again later in that same book (credit to Valorum):
"I, too, am curious. Put your spice-essence on the ledge beside Moneo. I will keep it."
Slowly, demonstrating by the steadiness of her movements that she intended no attack, Luyseyal reached beneath her gown and removed a small vial which glistened with an inner blue radiance. She placed the vial gently on the ledge. Not by any sign did she indicate that she might try something desperate.
"Truthsayer, indeed," Leto said.
So apparently the dying of the eyes is literally a saturation of the eye tissue with whatever chemical makes the spice blue.
