This will depend a lot on your conditions. To see a specific set of traits, you first need to isolate them completely. Over a couple of generations (ten maybe, maybe more or less depending on how many your bunch is) you will probably see they sort of look like they belong to the same family. Similar skin, similar hair shade, probably a certain range of eye color, etc. This is just the product of genes mixing and sort of homogenizing a bit over time.

Like someone mentioned, you could get a mutation, and if the mutation turns out to be either advantageous or simply dominant gene wise then you will eventually see it in most people.

Then finally you have adaptation by natural selection, and this will depend a lot on the type of pressure you subject this bunch of people to. Do they have access to electricity? Are they living completely in the dark? Any diseases affecting them or other creatures underground? Do they have access to space or is this limited? Food? You will likely see your surviving population becomes immune or resistant to underground pathogens, simply because those who can’t will die before reproducing. Similarly if you are short on nutrients or physical space, you will see them shrink over time. If your conditions are more extreme and they’re completely in the dark, you will probably have a population develop some sort of echolocation sense (and this shouldn’t take too long, blind people can develop this to a degree already), and their metabolism will adapt to a much lower vitamin D (unless they can acquire it from some abundant food source). Over an even longer period of time they will either lose their eyesight or if they have access to light (fire? A few narrow openings to the surface?) they will adapt to see much better in the dark. Those are the more obvious adaptations I can think of but there could be a lot more depending on so many factors.