While it’s true that 5e places a lot of the work on the DM, I think in that case your group just failed to read the rules properly. Darkvision is one of the more well-defined abilities in the game.
When six newbies struggle to figure it out, then it isn’t well-defined. Or at the very least isn’t well structured to find the definition quickly. I will die on this hill.
I think what you’re actually alluding to is that the books are poorly organized, which is indeed a common complaint. Darkvision is very clearly explained, but the explanation is “treat dim light as bright light and darkness as dim light”. In order to understand what that means mechanically, you then have to go find the section on light levels and obscurement (and then the rules on obscurement require you to read the blinded and invisible conditions). This isn’t necessarily a bad thing and is actually how most TTRPGs handle nested mechanics, but D&D notoriously has a really bad index in the PHB, which means it’s very hard to actually find the nested mechanics you need. Lots of other TTRPGs will give you a page reference or something when they reference rules found in other parts of the book. 5e doesn’t.
