Transparent Bootstrap Panel

Simply go for "bg-transparent" as mentioned in the link above; https://getbootstrap.com/docs/4.1/utilities/colors/

Such;

<div class="panel panel-primary">
  <div class="panel-heading">
    <h3 class="panel-title bg-transparent text-dark">Panel primary</h3>
  </div>
  <div class="panel-body bg-transparent text-white">
    Panel content
  </div>
</div>

And definitely be sure not to forget adding bootstrap.

.transparent{    
    background-color: rgba(0,0,0,0.15);
}

.panel{
    background-color: transparent;
    border: 1px #222;
}

<div class="panel panel-default">
     <div class="panel-heading">Título</div>
     <div class="panel-body transparent">                  
     </div>
</div>  

CSS

    body {
        background:#c33;
    }

    .panel-transparent {
        background: none;
    }

    .panel-transparent .panel-heading{
        background: rgba(122, 130, 136, 0.2)!important;
    }

    .panel-transparent .panel-body{
        background: rgba(46, 51, 56, 0.2)!important;
    }

HTML

<div class="panel panel-primary panel-transparent">
  <div class="panel-heading">
    <h3 class="panel-title">Panel primary</h3>
  </div>
  <div class="panel-body">
    Panel content
  </div>
</div>

Example