Dim Hero & Add Background to Nav on scroll
Bearing in mind that you already were able to abstract the functionality of toggling the class name into a separate function, I see other changes that can be made to clean up the code:


Toggle Method: Use the toggle() method of classList to shorten the function changeClass()

function changeClass(offset, clsName) {
    document.body.classList.toggle(clsName, window.scrollY &gt;= offset);
}

DOM access method: Use getElementById() to select the element with Id heroArea instead of querySelector(). While it may likely never be noticeable on a sample page this small, it generally works faster. Check out this Sitepoint forum
and this SO question and its answers (and related posts).
Combined event handlers: The scroll event handlers can be combined to a single function that calls changeClass() twice. 

window.addEventListener("scroll", function() {
    changeClass(bottomOfNav, 'fixed-nav');
    changeClass(middleHero, 'dim-hero');
});

Arrow functions: Because ecmascript-6 features like const are used, others like arrow functions could be used for the functions- e.g. 

const changeClass = (offset, clsName) =&gt;  document.body.classList.toggle(clsName, window.scrollY &gt;= offset);


Though some would argue that is too long for a single line, so brackets can be used:

const changeClass = (offset, clsName) =&gt;  {
    document.body.classList.toggle(clsName, window.scrollY &gt;= offset);
};

CSS: combine selectors for common styles: I noticed there are two selectors that both have the same background style - those can be combined to a single ruleset:

.fixed-nav nav,
.text-block-two {
  background: #333;
}



The changes have been applied to the sample code below.



const hero = document.getElementById("heroArea");
// Finds the top third of the element by adding the top of the element to the height of the element then divide by 3
const bottomOfNav = hero.offsetHeight / 3;
const middleHero = hero.offsetHeight / 2;
const changeClass = (offset, clsName) =&gt;  document.body.classList.toggle(clsName, window.scrollY &gt;= offset);
    
window.addEventListener("scroll", function() {
  changeClass(bottomOfNav, 'fixed-nav');
  changeClass(middleHero, 'dim-hero');
});
* {
  padding: 0; 
  margin: 0;
  font-family: sans-serif;
}

nav {
  position: fixed;
  width: 100%;
  left: 0;
  right: 0;
  color: #fff;
  padding: 15px;
  text-align: center;
  text-transform: uppercase;
  font-family: sans-serif;
  transition: .3s;
}

.fixed-nav nav,
.text-block-two {
  background: #333;
}

.hero {
  height: 100vh;
  background: black;
  opacity: 1;
  transition: .3s;
}

.dim-hero .hero {
  opacity: 0;
}

.text-block {
  height: 50vh;
  background: #555;
  color: #fff;
  font-size: 30px;
  text-align: center;
  padding: 40px;
}
&lt;body&gt;
  &lt;nav&gt;Navigation&lt;/nav&gt;
  &lt;div id="heroArea" class="hero"&gt;&lt;/div&gt;
  &lt;div class="text-block"&gt;Lorem ipsum, dolor sit amet consectetur adipisicing elit. Eos ipsum et, omnis sit vero ab doloremque quia dolores mollitia. Doloremque maxime dolores quo eius ea. Ad, reiciendis minus. Dolorum, hic.&lt;/div&gt;
  &lt;div class="text-block text-block-two"&gt;Lorem ipsum, dolor sit amet consectetur adipisicing elit. Eos ipsum et, omnis sit vero ab doloremque quia dolores mollitia. Doloremque maxime dolores quo eius ea. Ad, reiciendis minus. Dolorum, hic.&lt;/div&gt;
&lt;/body&gt;