Is there any idiom in English that describes someone's eyes' black pupils? I am especially interested in some positive idiom that would ascribe a characteristic of being deep in thinking, noble, kind, considerate, wise, etc. to such a person.
When we say someone is wide-eyed, we usually mean they've got their eyelids fully open, rather than that their pupils are dilated. But usually the two go hand-in-hand anyway, because both are simply signals that someone is interested or aroused (or maybe frightened, but it certainly means they're paying attention).
I don't think there will be an idiom for black pupils with the connotations specified, simply because most people don't associate dilated pupils with wisdom or the other "noble" qualities. The most common "idiom" I know is bedroom eyes, defined here as a way of looking at someone that shows you are sexually attracted to them. To me, that means they've got dilated pupils, but it's not that common in the first place, and some people confuse it with sleepy eyes anyway.
It's a commonplace observation that The eyes are the window of the soul, but again I think this alludes to honesty, warmth, and closeness rather than wisdom, etc.
It's also not uncommon to say/write of eyes being limpid pools. Many people understand this to mean dilated pupils (i.e. - big and black) because they don't know that the relatively rare word limpid actually means "clear/transparent". Again, sensual, not philosophical connotations.
TL;DR: Remarkable black pupils are sexy/naughty, not wise/good!