user11772: 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?

user1871: 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.  

11772: <p>Here are few screencaps from different sources:</p>

<ul>
<li><p>Bleach 170:</p>

<p><a href="https://i.stack.imgur.com/mnc1N.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/mnc1N.jpg" alt="Bleach 170"></a></p></li>
<li><p>Bleach 297, where 16:9 episode actually used 4:3 material in one reminiscence scene (they even animated the black bars) - poor source material?</p>

<p><a href="https://i.stack.imgur.com/LIkEt.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/LIkEt.jpg" alt="Bleach 297"></a></p></li>
<li><p>Xena the Warrior Princess:</p>

<p><a href="https://i.stack.imgur.com/X0nhu.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/X0nhu.jpg" alt="Xena"></a></p></li>
<li><p>Finally a Youtube video: <div class="youtube-embed"><div>
                <iframe width="640px" height="395px" src="https://www.youtube.com/embed/wMFPe-DwULM?start=0"></iframe>
            </div></div></p></li>
</ul>

<p>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. <strong>What is it and why is it there?</strong> Is it used for some kind of calibration?</p>
 1871: <p>The first anime pic is throwing me off, but it seems to be an artifact of how interlaced analog signals were <a href="https://en.wikipedia.org/wiki/480i" rel="nofollow noreferrer">structured</a> and displayed.</p>

<blockquote>
  <p>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)</p>
</blockquote>

<p><strong>Update</strong>: this answer at the electronics site gives the <a href="https://electronics.stackexchange.com/questions/98922/why-there-is-625-lines-pal-in-t-v-scanning-or-odd-lines">full picture</a>.  </p>
