Wine-Dark Sea in Homer

I had a university teacher of the grade known in German and Scandinavian universities as Lector, in Greek, who taught us Greek poetry and Homer.

He used to say Homer was using "wine dark" because he was giving us some kind of ... psychedelic expressionist description.

There is an actually far simpler answer than Staffan Fogelmark's.

I spilled some red wine on some of my papers, and now the stains are no more red but blue.

Red wines take their color from grape skin pigments called anthocyanins, ... Anthocyanins are also nature’s litmus test: Their color can be reflective of the pH of whatever they come into contact with. When anthocyanins are in an acidic environment (like wine), they're red, but if you introduce them to an alkaline solution (like water with a slightly elevated pH, or if cleaning products are added into the mix) they turn blue.

ASK DR. VINNY : What causes the blue tinge when I clean red wine out of my wineglass?
Nov 8, 2019
https://www.winespectator.com/articles/what-causes-the-blue-tinge-when-i-clean-red-wine-out-of-my-wineglass

So, presumably the phrase was fairly standard one for describing red wine stains on the Greek tunics known as χιτωνες. Any stain is darker than the white linen. The darkness or stain from wine after a while became blue in the washing process. As Dr. Vinny puts it:

You might also notice this phenomenon if you are trying to clean a wine spill out of your carpet or shirt: The stain might turn blue before (or instead of) disappearing.

Problem solved, next question!/HGL

PS, seems "dark" is only part of the translation ... but wine is there in the original./HGL