Shadal has a fascinating suggestion in his notes to the Arukh. He believes that we find "ohr" and "naghi" in reference to night in the Mishna and Gemara because it was a holdover from times (he mentions Zoroastrianism) where there was a belief in the god of light (and therefore good) and the god of darkness (and therefore evil). In the semitic languages, it became common for them to refer to night with the word "light", since they did not want to mention night as a symbol of evil. The Jews, speaking Hebrew, did not believe in these gods, but the language quirk remained nonetheless.
He says that after some years, he realized that the phrase "lashon sagi nahor" is actually evidence that he is correct, since if you think about it, we don't want to say about the blind person that they are blind, yet we don't have euphemisms for the shoteh, cheresh, etc. The reason they wanted a euphemism for the blind is that they only see darkness, and we don't want to say anything implying the lack of light. Very interesting.
A connection to this idea appears in Shadal's commentary to Genesis 1:4, where he suggests the Torah describes God as having separated light and dark because there were people who thought light and dark were two different gods...
ויבדל וגו': התקין שלא יתערבו זה עם זה, אלא כשיהיה אור לא יהיה חושך, וכשיהיה חושך לא יהיה אור, ואמר זה (אעפ"י שלא היה צריך לאמרו) מפני שלפי דעת הקדמונים אין החושך העדר האור בלבד, אבל היה גם הוא יש, כענין שנאמר יוצר אור ובורא חשך (ישעיה מ"ה ז'), וכן (איוב ל"ח י"ט) אי זה דרך ישכון אור וחשך אי זה מקומו, ולכך היו העמים מיחסים אלוה אחד לאור ואחד לחושך; ורצה הקב"ה להודיע לעמו כי הוא ית' אדון האור והחושך, ומאתו לבדו כל חוקותם.
