Forgot to include photos in original post:

kateholcombe.com shows a few solutions to consider.

I would cover it with a long piece of art

The suggestion from ilikefriday sounds lovely. If you decide on a dark wallpaper, perhaps there is a design in the paper that will inspire the art. Are there any wallpapers you’ve been considering?

That would drive me crazy also. You have a few options.

1. A blackout curtain that spans the area behind the bed. It's a long wall, though and a lot of fabric to cover a little window. It does make the room feel cozy, though.

An extra long roman shade (I would do either an inside mount that is right up against the glass, or an outside mount that covers the edges). Would have to be custom with that size. These don't seem to be blackout, but just showing an outside mount.

You can get cellular shades that are blackout - may be the most economical option. Actually roller blinds are more economical, but not sure you can get one in that size.

Or if you don't care about having daylight there, you could cover the window with foam core and hang a painting in front of it. It would have to be a big painting...

I was thinking this wallpaper (we love a dark & moody bedroom) but will have to rethink it depending on what we do with this window. @ffpalms

I like the art and shade suggestions mentioned above!

I love this cloud art, even without the dark wallpaper.

As an alternative, you could hang a rug behind the bed instead of art.

If you are covering it with patterned wallpaper you could install a closely fitting panel of MDF into the opening, painted white on the back, flush with the interior wall, and paper over it. You could install it on cleats or something reversible.