Same with the Mediterranean. I was puttering around Cinque Terre in a small motorboat at sunset some years back, and Homer’s epithet immediately came to mind.
There, the deep, dark sea contrasts even more starkly with the bright, crystal-clear waters along the shore. I suspect this was the distinction Homer was after, rather than trying to describe the specific hue for an audience that was likely closely familiar with the appearance of the sea to begin with.
Really dark purple wine can start to look bluish in the right environment, which could remind someone of a dark ocean on a cloudy day.
I’ve never lived near the Atlantic, only visited periodically, so probably unaware of the full spectrum of its appearance. I’ve had a lot of wine, though.