Black line in certain videos? Here are few screencaps from different sources:


Bleach 170:


Bleach 297, where 16:9 episode actually used 4:3 material in one reminiscence scene (they even animated the black bars) - poor source material?


Xena the Warrior Princess:


Finally a Youtube video: 
                
            


If you look closely, each of these frames contains a thin black line near the bottom, or in the case of the Youtube video, near the top. The line spans about 60% of the video width. What is it and why is it there? Is it used for some kind of calibration?
The first anime pic is throwing me off, but it seems to be an artifact of how interlaced analog signals were structured and displayed.


  For analog NTSC there is a total of 525 scanning lines of which
  originally 483 lines were visible (241.5 visible lines + 21 lines of
  vertical blanking per field)


Update: this answer at the electronics site gives the full picture.