My understanding is that they don't distinguish blue and green as separate colours. Kind of like pink is just light red and not every language has a separate word for pink and red.

So I started skimming the article and:

Gladstone noticed Homer described the sea color as “wine-dark,”

I KNEW IT

It's the fuckin Greek thing again.

Listen, I'll tell a story in three parts.

1. People started saying a few years ago that ancient Greeks didn't make a distinction between blue and red, because look at what Homer said, and the sea is obviously not wine colored. Or maybe they had blue wine? But anyway we think they thought they were the same color.
2. I always thought it was a bunch of shit. Blue and red are different. They're so clearly different that some people morphed the whole thing into a lack of distinction between blue and green, since that makes quite a bit more sense sense and there definitely are cultures that don't have different names for similar colors like blue and green that English has different words for. But anyway, the issue here was blue and red, and to me, I was always convinced that it was a bunch of shit.
3. And look - I WAS RIGHT. I was all ready to write up #1 and #2 in answer to your question and agreeing with you, but without the punchline, but just now I looked it up to be able to bitch about it a little more effectively, and learned that smart people have in the meantime figured out that the whole "wine dark sea" thing was talking about the sea being dark in brightness, like wine is dark, i.e. not light and clear and happy like the Mediterranean often is. But still colored blue presumably. So, not a bunch of surprising and confusing stuff about "blue=red" that sounds suspiciously like nonsense, but something that's perfectly sensible.

TL;DR you are correct. Blue is blue and always was. The people who are telling you blue and red used to be the same are probably just confused, and if Homer comes into the equation then that's a telltale sign that they are absolutely confused and you don't have to listen to them.

Depending on where you are, water can definitely look very blue or even vivid green.

There's been research that language shapes how we perceive the world around us. Because there was no word for "blue" there was no concept of blue, the color still existed but their brains just lumped it into "green". Sight works by the visual centers brain taking data from the eyes, throwing most of it out, then building a model which is what the rest of the brain gets to actually "see". That's why optical illusions work.

A commonly cited source for language shaping our perception of color is Jules Davidoff's studies on the Himba tribe. The Himba have no word for blue, and they struggled to pick out the blue square from this color wheel. However, they do have many distinctions for shades of green so when given this color wheel they could easily pick out the square that's a different shade of green (and yes I opened it in MSpaint to check and one of the green squares is a different shade.)

It's just terminology. People make a much bigger deal out of it than it is.

I think part of this phenomena is that the color of the sky on a clear day just wasn't considered as a color, it was 'blank', the absence of clouds, stars or sunset. Kind of like how water has a flavor but we consider it to be neutral.

Homer referred to the ocean as "wine dark", if you've seen pictures of the Mediterranean around Greece, that shit's as Blue as Blue can be, so what it probably boiled down to is that they could "see" blue, but they always understood it as a blackish color or Green if they had distinguished that by that point.

Basically, there wasn't a lot of necessity use in distinguishing blue, so most people didn't until recently.