If you can't do anything else there's a variety of window film and installers. Personally I dislike traditional blinds as I find they often don't cut enough light, or leave obnoxious holes of light through.
If you need to cut solar heat transfer you'll want metalized films, but if all you want is darker you can definitely do that with a variety of film with different grades of darkness.
Of course, there are more advanced slide up/slide down blinds as well.