Using bootstrap cards as a hyperlink

Its because <a> tags has a default blue color from user agent browser. Try to add a class to the link and set color:inherit to that class

a.custom-card,
a.custom-card:hover {
  color: inherit;
}
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css">

<div class="card" style="width: 15rem; display: inline-block">
  <img class="card-img-top" src="https://placeholdit.imgix.net/~text?txtsize=33&txt=318%C3%97180&w=318&h=180" alt="Card image cap">
  <div class="card-body">
    <h5 class="card-title">Normal card</h5>
    <p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
  </div>
</div>

<a href="" class="custom-card">
  <div class="card" style="width: 15rem; display: inline-block">
    <img class="card-img-top" src="https://placeholdit.imgix.net/~text?txtsize=33&txt=318%C3%97180&w=318&h=180" alt="Card image cap">
    <div class="card-body">
      <h5 class="card-title">Wrapped with a tag</h5>
      <p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
    </div>
  </div>
</a>

Instead of wrapping the .card in a <a>, you could use a <a> as the card element instead of a <div>.

This will allow you to easily override the CSS to remove the default <a> styles:

a.card,
a.card:hover {
  color: #212529;
  text-decoration: none;
}
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css">

<div class="card" style="width: 15rem; display: inline-block">
  <img class="card-img-top" src="https://placeholdit.imgix.net/~text?txtsize=33&txt=318%C3%97180&w=318&h=180" alt="Card image cap">
  <div class="card-body">
    <h5 class="card-title">Normal card</h5>
    <p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
  </div>
</div>

<a href="#" class="card" style="width: 15rem; display: inline-block">
  <img class="card-img-top" src="https://placeholdit.imgix.net/~text?txtsize=33&txt=318%C3%97180&w=318&h=180" alt="Card image cap">
  <div class="card-body">
    <h5 class="card-title">Wrapped with a tag</h5>
    <p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
  </div>
</a>

Very simple with Stretched link

<div class="card" style="width: 18rem;">
  <img src="..." class="card-img-top" alt="...">
  <div class="card-body">
    <h5 class="card-title">Card with stretched link</h5>
    <p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
    <a href="#" class="btn btn-primary stretched-link">Go somewhere</a>
  </div>
</div>

If you are using Bootstrap 4.3 you don't have to wrap card with <a> tag, but instead put it inside card-body, and use stretched-link class. This is much cleaner way.

Visit https://getbootstrap.com/docs/4.3/utilities/stretched-link/ for more details.

If you can't use Bootstrap 4.3 this is styling for stretched-link class:

.stretched-link::after {
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    z-index: 1;
    pointer-events: auto;
    content: "";
    background-color: rgba(0,0,0,0);
}

Here is the example:

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css">

<div class="card" style="width: 15rem; display: inline-block">
  <img class="card-img-top" src="https://placeholdit.imgix.net/~text?txtsize=33&txt=318%C3%97180&w=318&h=180" alt="Card image cap">
  <div class="card-body">
    <h5 class="card-title">Normal card</h5>
    <p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
  </div>
</div>


<div class="card" style="width: 15rem; display: inline-block">
  <img class="card-img-top" src="https://placeholdit.imgix.net/~text?txtsize=33&txt=318%C3%97180&w=318&h=180" alt="Card image cap">
  <div class="card-body">
    <h5 class="card-title">Alternative</h5>
    <p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
    <a href="#" class="stretched-link"></a>
  </div>
</div>