To me, Stygian blue doesn't look like blue on top of black. It looks like black that glows blue, which doesn't make sense in the real world. I think it is fairly described as a new color—I would be quite unsettled if I encountered it in real life.
Of course, colors are a hallucination our brain produces, so perhaps different brains deal differently with an unusual experience like Stygian blue.
> I think it is fairly described as a new color—I would be quite unsettled if I encountered it in real life.
I don't feel this follows. There are a lot of things that would unsettle me if I saw them, like if someone gave off a visible aura. Heck, I even found a "black flame" a bit unsettling, and I saw a literal video of it on YouTube (look it up if you don't know what I'm referring to). I'd feel similarly if I saw a transparent human too. The feeling you get - or the fact that you haven't seen something visually similar before - doesn't really imply it's a new color, I think!
I get more of a black in the middle and blue glow around the edges. Also not sure it qualifies as a new color. To me, it's more like an interesting illusion that combines black and blue.
Of course, colors are a hallucination our brain produces, so perhaps different brains deal differently with an unusual experience like Stygian blue.