What does this black bar represent in my iPod's Storage Capacity Indicator?
To my belief, that indeed is actually just a drop shadow. Since you're actually not on the newest version of iTunes, I can't verify that, since it looks different now, but here are a few photos:





iTunes 12 (latest version)





Version that looks the same as yours also showing shadow





Older version also displaying drop shadow



With that said, I could still be wrong. Check and make sure it isn't any of these categories:


Audio: Songs, audio podcasts, audiobooks, voice memos, and ringtones
Video: Movies, music videos, and TV shows 
Photos: Camera Roll content, Photo Stream, and Photo Library
Apps: Installed apps
Books: iBooks books and PDF files 
Documents &amp; Data: Safari Offline Reading List, files created within apps, and app content like contacts, calendars, messages, and emails (and their attachments)
Other:
Settings, Siri voices, system data, and cached files