In the DUNE universe, what exactly is SPICE?

A drug.

It was a chemical, in the water and soil (and got into plants) that was animal based (from the worms that died drowning, thereby contaminating the base water supply on the planet). It was everywhere, but the blue eyes were a physical change based on the amount in someone's body.

Different things do physical stuff to people. Too much sun gives sunburn (and skin cancer). Smoking can cause lung issues. Drinking too much lowers liver function significantly.

worm poop....

It's a drug and the most valuable commodity in that universe. People who have the whites of their eyes turn blue show that they consume spice.

a drug that gives the user a longer life span, greater vitality, and heightened awareness; it can also unlock prescience in some humans, depending upon the dosage and the consumer's physiology.[2] This prescience-enhancing property makes safe and accurate interstellar travel possible.[2] Melange comes with a steep price, however: it is addictive, and withdrawal is fatal. Spice addiction, which everyone who took spice became addicted was marked by the blue within blue eyes. Vanity however caused almost everyone to wear contact lenses that changed to color of their blue eyes. The race of humans "native" to Dune did not disguise their eyes.

It's "a drug that gives the user a longer life span, greater vitality, and heightened awareness; it can also unlock prescience in some humans, depending upon the dosage and the consumer's physiology. This prescience-enhancing property makes safe and accurate interstellar travel possible. [It] comes with a steep price, however: it is addictive, and withdrawal is fatal."

The blue eyes are a sign of long-term use and addiction.

Melange, otherwise known as Spice, is described as a heavy brown powder which smells like cinnamon and glows blue. Its described as a drug.

Its created when water floods into the excretions of small/baby Sand Worms.

Repeated use mutates the sclera, cornea, and iris of the user to a dark shade of blue. THeres no real explanation beyond it have physiological side effects from continuous use.

Long time since I read the book; from memory it is a 100% addictive drug. White of eyes turn blue.

Further thought: The spice is called 'melange', produced by huge, dangerous sand worms.