css transition opacity fade background

Wrap your image with a span element with a black background.

.img-wrapper {
  display: inline-block;
  background: #000;
}

.item-fade {
  vertical-align: top;
  transition: opacity 0.3s;
  opacity: 1;
}

.item-fade:hover {
  opacity: 0.2;
}

<span class="img-wrapper">
   <img class="item-fade" src="https://via.placeholder.com/100x100/cf5" />
</span>

It's not fading to "black transparent" or "white transparent". It's just showing whatever color is "behind" the image, which is not the image's background color - that color is completely hidden by the image.

If you want to fade to black(ish), you'll need a black container around the image. Something like:

.ctr {
    margin: 0; 
    padding: 0;
    background-color: black;
    display: inline-block;
}

and

<div class="ctr"><img ... /></div>