I tried searching the web for various answers and tried everything that solved other people problem, but my CSS just doesn't want to cooperate.
Here you can find a JSFiddle where I recreated the issue: I would like the side navigation to be visible on the black sections too.
HTML
<div class="sidenav">
  <a href="#" class="selected">Section 1</a>
  <a href="#">Section 2</a>
  <a href="#">Section 3</a>
</div>
<div class="content">
  <div>
    <p>Some text<p>
  </div>
  <div class="blk">
    <p>Some text<p>
  </div>
  <div>
    <p>Some text<p>
  </div>
</div>
CSS
.sidenav {
  width: auto;
  position: fixed;
  z-index: 1;
  top: 150px;
  padding: 8px 0;
}
.sidenav a {
  clear: both;
  float: left;
  position: relative;
  left: -20%;
  padding: 10px;
  margin-bottom: 15px;
  transition: 0.3s;
  text-decoration: none;
  text-align: right;
  font-size: 24px;
  border-style: solid;
  border-width: 2px;
  border-color: #0D0D0D;
  border-radius: 0 5px 5px 0;
  color: #0D0D0D;
  mix-blend-mode: difference;
}
.blk {
  background-color: #0d0d0d;
  color: #ffffff;
}