Will you be using it as a study? I'd put a small desk or table in there, with a lamp that I can turn on from the front door entrance. If the house isn't wired for a switch in the foyer, put the lamp on a remote control system.

It looks like a dark hole because it is a dark hole. I see two options. One is to light it up even when you aren't using it (as Maddielee suggested). The other is to cover it up. Louvered door would allow air circulation while still covering the opening. If you google "ventilated door" you'll find some other options that would do the same thing.

How you will use it drives how you make it look. If you will use it as a work area, storing home records etc, I'd build in storage on the short wall, so that when you are working you don't have your back to the rest of the home. Then perhaps shallow shelves/cabinets across the long wall, less than 12", leaving space in the middle for nice colorful piece of artwork. A cozy lamp on those shelves, but of course a task light or lamp on the work surface.

I don't think it has to be bright, it can look like a little hideaway, but it should have some interest and a central eye-catcher since you see it from the dining room and as you come out of the bedroom.

Do you plan to use color or paint the condo? I don't know how you feel about accent walls, but painting the nook's exterior wall a different color than the interior could look intriguing.

If you are not going to use it as a study you could make it a sort of pantry but leaving space to get to the bathroom. I would use some lamps.

Adding louvered doors is a great idea. Doors with large slats will look beachy, allow for ventilation and solve the black hole problem.

I'd embrace it-it's a small space. I'd go there dark, in terms of paint/wallpaper, and use mirrors and great lighting..several layers of such if possible..and bring metallics, brights or white accents-whatever speaks to you and is more of your style

if I put there a small desk, bookshelves, etc-I'd think of how to incorporate it all together. It can be a lot of fun, to plan a small special spase like that

When one doesn't have light one creates it..

PS your new condo looks magnificent btw. Congrats!!

You probably don't want to invest in any additional construction right now, but I would probably reconfigure that area along with the adjacent storage to increase the closed storage area and make the rest a hall to the powder room, perhaps lined with display shelves or bookcases.